Inclusion Criteria:

          -  General Eligibility Criteria (All Parts)

          -  Written informed consent - a signed informed consent and/or assent (as age
             appropriate) for study participation including PK sampling will be obtained according
             to institutional guidelines.

          -  Male or female between one month and <18 years of age (inclusive) at the time of
             signing the informed consent form (Part C and Part D between 12 months and <18 years
             of age, inclusive).

          -  Must have a disease that is relapsed/refractory to all potentially curative standard
             treatment regimens or must have a current disease for which there is no known curative
             therapy, or therapy proven to prolong survival with an acceptable quality of life.

          -  Prior therapy: The subject's disease (i.e. cancer, neurofibromatosis type 1 [NF-1]
             with plexiform neurofibroma [PN], or Langerhans cell histocytosis [LCH]) must have
             relapsed after or failed to respond to frontline curative therapy or there must not be
             other potentially curative treatment options available. Curative therapy may include
             surgery, radiation therapy, chemotherapy, or any combination of these modalities. All
             subjects must have recovered to grade <=1 from the acute toxic effects of all prior
             chemotherapy, immunotherapy, or radiotherapy prior to enrollment. Prior therapy
             includes; myelosuppressive chemotherapy, differentiating agents/ biologic response
             modifiers (small molecules, antibodies, viral therapies) (anti-cancer agent),
             non-myelosuppressive anticancer agents, investigational agent, radiation therapy, stem
             cell transplantation or infusion, number of prior treatment regimens, colony
             stimulating factors, corticosteroids.

          -  Performance score of >=50% according to the Karnofsky/Lansky performance status scale.

          -  Females of child-bearing potential must be willing to practice acceptable methods of
             birth control. Additionally, females of childbearing potential must have a negative
             serum pregnancy test within 7 days prior to start of study drugs, throughout treatment
             period and for 4 months after last dose of study drugs.

          -  Must have adequate organ function as defined by the following values: renal function -
             24 hr creatinine clearance (revised Schwartz formula), or radioisotope glomerular
             filtration rate (GFR) >=60 milliliter (mL) per minute per 1.73 meter square
             (mL/min/1.73m^2); or a serum creatinine <=upper limit of normal (ULN) for age and
             gender; liver functions as bilirubin (sum of conjugated + unconjugated) <=1.5 x ULN
             for age, alanine aminotransferase (ALT) <=2.5 x ULN; for the purposes of enrollment
             and toxicity monitoring the ULN for ALT will be 45 unit per liter (U/L); cardiac
             function - corrected QT (QTcB) interval <480 milliseconds (msec), left ventricular
             ejection fraction (LVEF) >=lower limit of normal (LLN) by ECHO.

          -  Able to swallow and retain enterally (per oral [PO] or nasogastric or gastric tube)
             administered medication and does not have any clinically significant gastrointestinal
             abnormalities that may alter absorption such as malabsorption syndrome or major
             resection of the stomach or bowels.

          -  Adequate Blood Pressure Control defined as: Blood pressure <= the 95th percentile for
             age, height, and gender.

          -  French subjects: In France, a subject will be eligible for inclusion in this study
             only if either affiliated to or a beneficiary of a social security category.

          -  Specific Eligibility Criteria, Part A

          -  Subjects must meet general eligibility criteria.

          -  For the initial dose escalation to identify the maximum tolerable or PK target dose,
             age between 2 years and <18 years (inclusive) at the time of signing the informed
             consent form. Children < 2 years of age will be enrolled once the age specific
             expansion cohorts are open.

          -  Histologically confirmed solid tumors, which may include but are not limited to
             rhabdomyosarcoma and other soft tissue sarcomas, Ewing sarcoma family of tumors,
             osteosarcoma, neuroblastoma, Wilms' tumor, hepatic tumors, germ cell tumors, primary
             brain tumors, NF-1 associated PF and LCH. In subjects with brain stem gliomas the
             requirement for histological confirmation can be waived if a biopsy was not performed.
             For plexiform neurofibromas, histologic confirmation of tumor is not necessary in the
             presence of consistent clinical and radiological findings, but should be considered if
             malignant degeneration of a PN is clinically suspected.

          -  Measurable or evaluable tumors. Subjects with neuroblastoma that is only detectable by
             Meta-iodobenzylguanidine (MIBG) scan are eligible. Subjects with neuroblastoma that is
             only detected by bone marrow aspirate/biopsy or elevated homovanillic acid /
             vanillylmandelic acid (HVA/VMA) are not eligible.

          -  Adequate bone marrow function defined as absolute neutrophil count (ANC)
             >=1000/microliter, hemoglobin >=8.0 gram per deciliter (g/dL) (may receive red blood
             cell transfusions), platelets >=75,000/ microliter (transfusion independent, defined
             as not receiving platelet transfusions within a 7 day period prior to enrollment).

          -  Specific Eligibility Criteria, Part B

          -  Subjects must meet general eligibility criteria. The specific eligibility criteria
             listed here will apply to subjects enrolling to different cohorts of Part B.

          -  Tumor tissue (archived or fresh) is required and must be available to be shipped to
             GSK or site specific laboratory.

          -  Solid tumor cohort (B1) specific criteria

          -  B1: Refractory or relapsed neuroblastoma

          -  B2: Recurrent or unresectable low grade gliomas with BRAF tandem duplication with
             fusion

          -  B3: Neurofibromatosis Type -1 associated plexiform neurofibromas (NF-1 with PN) that
             are unresectable and medically significant.

          -  B4: BRAF V600 mutant tumors.

          -  Specific Eligibility Criteria, Part C - Subjects must meet general eligibility
             criteria.

          -  Tumors that have been documented by CLIA or equivalent certified laboratory test to
             harbor BRAF V600 mutation at diagnosis or relapse

          -  Measurable or evaluable disease

          -  Adequate bone marrow function

          -  Specific Eligibility Criteria, Part D - Subjects must meet general eligibility
             criteria

          -  Measurable or evaluable disease

          -  Recurrent or refractory BRAFV600 mutant LGG or LCH tumors

          -  Adequate bone marrow function

        Exclusion Criteria:

          -  Lactating or pregnant female.

          -  History of another malignancy including resected non-melanomatous skin cancer.

          -  Subjects with NF-1 associated optic pathway tumors are excluded if they are actively
             receiving therapy for the optic pathway tumor or do not meet criteria for PN or
             malignant solid tumor.

          -  Subjects with a history of NF-1 related cerebral vascular anomaly (such as Moyamoya).

          -  Subjects with NF-1 actively receiving therapy for the optic pathway tumor.

          -  Subjects with NF-1 and only PN lesions that cannot be evaluated by volumetric analysis
             (only applicable to Part B).

          -  Any serious and/or unstable pre-existing medical, psychiatric disorder or other
             conditions that could interfere with subject's safety, obtaining informed consent or
             compliance to the study procedures.

          -  Any prohibited medication(s), currently used or expected to be required.

          -  Any medications for treatment of left ventricular systolic dysfunction.

          -  Part B, Part C and Part D only: Previous treatment with dabrafenib or any BRAF
             inhibitor, trametinib or another MEK inhibitor, or and Extracellular signal-regulated
             kinase inhibitor (exception: prior treatment with sorafenib is permitted). Patients
             who have received prior dabrafenib or another BRAF inhibitor may enrol into Part B4.
             Patients who have had prior dabrafenib or BRAF inhibitor therapy may enroll in Part C
             or Part D if they have had prior benefit to dabrafenib or BRAF inhibitor monotherapy,
             as determined by the investigator.

          -  Administration of an investigational study treatment within 30 days preceding the
             first dose of study treatment(s) in this study.

          -  Have a known immediate or delayed hypersensitivity reaction or idiosyncrasy to drugs
             chemically related to study treatment or excipients that contraindicate their
             participation.

          -  Current active liver or biliary disease (with the exception of Gilbert's syndrome or
             asymptomatic gallstones, or liver metastases).

          -  History of hepatic sinusoid obstructive syndrome (venoocculsive disease) within the
             prior 3 months.

          -  History of heparin-induced thrombocytopenia.

          -  History of interstitial lung disease or pneumonitis.

          -  History of or current evidence of retinal vein occlusion (RVO).

          -  For subjects with solid tumors that are not primary central nervous system (CNS)
             tumors or NF-1 associated plexiform neurofibromas subjects with symptomatic or
             untreated leptomeningeal or brain metastases or spinal cord compression are excluded.
             NOTE: Subjects previously treated for these conditions that have had stable CNS
             disease (verified with consecutive imaging studies) for >3 months, are asymptomatic
             and are not currently taking corticosteroids, or are on stable dose or decreasing of
             corticosteroids for at least 7 days prior to enrolment are permitted.

          -  A history of known Hepatitis B Virus (HBV), or Hepatitis C Virus (HCV) infection.
             Subjects with laboratory evidence of cleared HBV and HCV infection may be enrolled.

          -  Unresolved toxicity of National Cancer Institute Common Terminology Criteria for
             Adverse Events, version 4.0 (NCI CTCAE v4.0) Grade 2 or higher from previous
             anti-cancer therapy, except alopecia.

          -  Presence of active gastrointestinal (GI) disease or other condition that will
             interfere significantly with the absorption of drugs.

          -  A history or evidence of cardiovascular risk including: a QT interval corrected for
             heart rate using the Bazett's formula (QTcB) >=480 msec; a history or evidence of
             current clinically significant uncontrolled arrhythmias (clarification: Subjects with
             atrial fibrillation controlled for >30 days prior to dosing are eligible); a history
             of acute coronary syndromes (including myocardial infarction or unstable angina),
             coronary angioplasty, or stenting within 6 months prior to randomization; a history or
             evidence of current >=Class II congestive heart failure as defined by the New York
             Heart Association (NYHA) guidelines; subjects with intra-cardiac defibrillators;
             abnormal cardiac valve morphology (>=grade 2) documented by echocardiogram (subjects
             with grade 1 abnormalities [i.e., mild regurgitation/stenosis] can be entered on
             study). Subjects with moderate valvular thickening should not be entered on study.
             Subjects with prosthetic valves can be considered eligible provided they meet the
             criteria as stated above; Treatment refractory hypertension defined as a blood
             pressure of systolic >140 millimeter of mercury (mmHg) and/or diastolic >90 mmHg (or
             above 95th age-specific percentile listed in protocol), which cannot be controlled by
             anti-hypertensive therapy.