MAPK Inhibition Combined With Anti-PD1 Therapy for BRAF-altered Pediatric Gliomas

Inclusion Criteria: Cohort A Only: * Patients with histologically confirmed diagnosis of pediatric high- or low-grade glioma harboring a KIAA1549-BRAF fusion: Low-grade glioma that is recurrent or progressive OR High-grade glioma that is newly diagnosed or recurrent OR * Patients with NF1-associated gliomas or NF1-altered glioma: Low-grade glioma that is recurrent or progressive OR High-grade glioma that is newly diagnosed or recurrent OR Transforming glioma that is newly diagnosed or recurrent Cohort B Only: * Patients with histologically confirmed diagnosis of pediatric low-grade glioma harboring a BRAFV600 mutation that is recurrent or progressive OR * Patients with histologically confirmed diagnosis of non-brainstem pediatric high-grade glioma harboring BRAFV600 mutation that is newly diagnosed, recurrent, or progressive All Cohorts: * Patients must be ≥1 and ≤26 years of age at the time of enrollment. * Patients must have a performance status of Karnofsky \>50% for patients \>16 years old and Lansky \>50% for patients \<16 years old. * Patients must have adequate organ and bone marrow function * The effects of dabrafenib, trametinib, and nivolumab on the developing human fetus are unknown. For this reason, patients of childbearing potential (POCBP) and patients with sperm-producing reproductive capacity (PWSPRC) must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) from time of informed consent for the duration of study participation and for 30 days following completion of therapy. POCBP must have a negative pregnancy test. * Patients with neurological deficits that are stable for a minimum of 1 week prior to enrollment are eligible. Note: A baseline detailed neurological exam should clearly document the neurological status of the patient at the time of enrollment on the study. \- Patients who are receiving dexamethasone must be on a stable or decreasing dose for at least 1 week prior to enrollment. Total dexamethasone dose at time of enrollment must be less than or equal to 2 mg/day total or 0.5 mg/kg/day, whichever is smaller. LGG Only * Patients must have received a prior BRAF inhibitor (first or second generation), MEK inhibitor, or a combination. The response to this therapy must be known and information provided at study enrollment. * Patients must have recovered from acute treatment-related toxicities (defined as \<Grade 1, excludes alopecia) prior to entering this study. HGG Only * Patients must have received prior radiotherapy \>12 weeks prior to enrollment. * Patients must have recovered from acute treatment-related toxicities (defined as \<Grade 1, excludes alopecia) prior to entering this study. * NF1 patients with transforming gliomas and high-grade gliomas are eligible regardless of prior systemic therapy. * Patients who have received prior radiation therapy must have experienced progression post-radiation OR have measurable disease defined as residual tumor \>1cm in at least one dimension Exclusion Criteria: * Patients with disseminated disease. * Patients who have had prior radiation therapy \<12 weeks prior to registration. * Patients who have not recovered from adverse events due to prior anti-cancer therapy (i.e., have residual toxicities \> Grade 1) (with the exception of alopecia). * Patients who receiving any other investigational agents. Note: There will be a 21-day washout period for all chemotherapeutic agents, a washout period of two half-lives for any targeted agents (e.g., MAPK inhibitors), and/or a washout period of 4 weeks for any antibody therapies (e.g., bevacizumab). * Patients who have a history of allergic reactions attributed to compounds of similar chemical or biological composition to dabrafenib, trametinib, or nivolumab. * Patients who have received MAPK inhibitor and checkpoint blockade combination therapy. Note: Patients may have received MAPK inhibitor monotherapy or checkpoint blockade monotherapy. * Patients who previously discontinued BRAF inhibitor (type 1 inhibitor or dimer inhibitor, such as, DAY101), MEK inhibitor, or the combination because of grade 3 or higher toxicity or clinically significant grade 2 toxicity requiring discontinuation of therapy are not eligible. * Patients with the following: * Known autoimmune disorders * Immune disorders * Immunodeficiencies * Patients with Crohn's disease, ulcerative colitis, or other inflammatory bowel disease. * Patients with active pancreatitis or history of pancreatitis within the last 3 months. * Patients with active interstitial lung disease (ILD)/pneumonitis or a history of ILD/pneumonitis requiring treatment with systemic steroids. * Patients who have a known active Human Immunodeficiency Virus (HIV), Hepatitis B, or Hepatitis C infection are ineligible. Patient must have documented evidence of negative tests for the presence of HIV, Hepatitis B surface antigen, and Hepatitis C (anti-HCV antibody OR Hep C RNA-qualitative). * Patients who have received a major surgical procedure ≤ 28 days of beginning study treatment, or minor surgical procedures (including VP shunt placement or stereotactic biopsy of the tumor) ≤ 7 days are not eligible. * Patients who are taking herbal preparations. These medications include but are not limited to St. John's wort, kava, ephedra (ma hung), gingko biloba, dehydroepiandrosterone (DHEA), yohimbe, saw palmetto, and ginseng. Cannabis products of any type are not allowed throughout the study. Patients should stop using these herbal medications or cannabis products 7 days prior to enrollment. * Patients who are pregnant. Patients of childbearing potential must have a negative serum or urine pregnancy test. (If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required.) * Patients who are lactating (unless they have agreed to not breastfeed). Breastfeeding patients are excluded from this study due to risks of fetal and teratogenic adverse events as