Phase I Trial of TURALIO(R) (Pexidartinib, PLX3397) in Children and Young Adults With Refractory Leukemias and Refractory Solid Tumors Including Neurofibromatosis Type 1 (NF1) Associated Plexiform Neurofibromas (PN) and Tenosynovial Giant Cell Tumor ...

Background: \- Some people with cancer have solid tumors. Others have refractory leukemia. This may not go away after treatment. Researchers want to see if a drug called TURALIO(R) can shrink tumors or stop them from growing. Objectives: \- To find the highest safe dose and side effects of TURALIO(R). To see if it helps treat certain types of cancer. Eligibility: \- People ages 3-35 with a solid tumor or leukemia that has returned or not responded to cancer therapies. Design: * Individuals will be screened with: * Medical history * Physical exam * Blood and urine tests * Heart tests * Scans or other tests of the tumor * Individuals will take TURALIO(R) as a capsule once daily for a 28-day cycle. They can do this for up to 2 years. * During the study, participants will have many tests and procedures. They include repeats of the screening tests. Individuals will keep a diary of symptoms. * Individuals with solid tumors will have scans or x-rays. * Individuals with leukemia will have blood tests. They may have a bone marrow sample taken. * Some individuals may have a biopsy. * When finished taking TURALIO(R), individuals will have follow-up visits. They will repeat the screening tests and note side effects.

* INCLUSION CRITERIA: * Diagnosis: * Individuals must have recurrent or refractory solid tumors or acute leukemia (limited to AML or ALL) or have been intolerant of prior therapies, confirmed by the Laboratory of Pathology, NCI, e.g., solid tumors including rhabdomyosarcoma, Ewing sarcoma, soft tissue sarcomas. These may include primary neoplasms of the central nervous system, such as high-grade (WHO grade III-IV) glioma. Individuals with diffuse intrinsic pontine glioma (DIPG) or optic pathway glioma are exempt from histologic verification. For DIPG typical MRI findings must be present which include hypo- or isointense on T1-weighted imaging, hyperintense on FLAIR or T2-weighted imaging, epicenter in the pons in the face of a typical clinical presentation. Optic pathway gliomas are located in the optic pathway and are typically hypo- or iso-intense on T1 and hyperintense on T2-weighted images. * In addition, individuals with NF1 and with malignant peripheral nerve sheath tumor (MPNST). * Individuals must have relapsed after or be refractory to effective standard therapies. There are no limits on number of prior therapeutic regimens. * Disease status: Individuals with refractory solid tumors including patients with NF1 and MPNST must have evaluable disease, patients with leukemia must have measurable or evaluable disease at the time of enrollment, which may include any evidence of disease including minimal residual disease detected by flow cytometry. * Age \>= 3 and \= 0.55 m\^2): * Ability of subject or Legally Authorized Representative \[LAR\] (the parent/guardian if subject is a minor) to understand and the willingness to sign a written informed consent document. * Individuals must be able to swallow capsules. * Performance Status: Karnofsky \>= 50% for patients \> 16 years of age and Lansky \>= 50% for patients \= 50% radiation of pelvis; \>= 14 days from whole brain radiation, craniospinal radiation, or targeted radiation to CNS tumors. At least 42 days must have elapsed if other substantial BM radiation. * HSCT: \>= 56 days from stem cell transplant with no evidence of active graft vs. host disease; must be off immunosuppressive therapy for at least 4 weeks and have no active graft-versus-host disease (GVHD) at the time of entry onto this trial. * Surgery: \>= 14 days from surgery * Others: \>= 7 days from last dose of short active hematopoietic growth factors, i.e. filgrastim, \>= 14 days for long-acting, i.e. pegfilgrastim. * Steroids: Individuals with CNS tumors who are managed with steroids are eligible if they have no worsening neurologic deficits and are on a stable or decreasing dose of corticosteroids for greater than or equal to 7 days prior to registration. Individuals with leukemia receiving corticosteroids or hydroxyurea are eligible provided that the corticosteroids are not being used to manage GVHD and there has been no increase in corticosteroid of hydroxyurea dose for 7 days prior to starting TURALIO(R). -Individual must have adequate hematologic, hepatic, and renal function, defined by: * Absolute neutrophil count \>= 1.5 x 10\^9/L * Hemoglobin \> 10 g/dL * Platelet count \>= 100 x 10\^9/L * AST and ALT \