Key Inclusion Criteria:
- Age: >=12 months and <18 years of age at the time of signing the informed consent form.
- BRAF V600 mutation-positive tumor as confirmed in a Clinical Laboratory Improvement Amendments (CLIA)-approved laboratory or equivalent (the local BRAF testing may be subject to subsequent verification by centralized testing; centralized testing can confirm V600E and V600K mutations only).
- Must have adequate organ function
Key Exclusion Criteria:
- Part 2 ONLY: Previous treatment with dabrafenib, another RAF inhibitor, or a mitogen-activated protein kinase (MEK) inhibitor (exception: prior treatment with sorafenib is permitted).
- Malignancy OTHER than the BRAF mutant malignancy under study.
- Had chemotherapy or radiotherapy within 3 weeks prior to administration of the first dose of study treatment.
- Autologous or allogeneic stem cell transplant within 3 months prior to enrolment [NOTE: subjects with evidence of active graph versus host disease are excluded].
- History of myocardial infarction, severe or unstable angina, peripheral vascular disease or familial QTc prolongation.
- Subjects with abnormal cardiac valve morphology (>=grade 2) documented by echocardiogram
- Subjects with moderate valvular thickening.
- Known, uncontrolled cardiac