Patients must be > 12 months and < 18 years of age at the time of enrollment

Patients must have a body surface area of > 0.3 m^2 at the time of enrollment

Existing or recurrent desmoid tumor that is deemed not amenable to surgery without significant morbidity and progressed by >= 10% as assessed by RECIST version (v)1.1 within the 6-month period prior to study enrollment

• Patients must have had histologic verification of the desmoid tumor
• Patients must have measurable disease by RECIST v1.1 criteria
• Patient must have received at least one prior course of systemic therapy for desmoid tumor

Patients must have a Lansky (for patients =< 16 years of age) or Karnofsky (for patients > 16 years of age) performance status score of >= 50. Patients who are unable to walk because of paralysis, but who are up in a wheelchair, will be considered ambulatory for the purpose of assessing performance score

Patients must have fully recovered from the acute toxic effects of all prior chemotherapy, immunotherapy, surgery or radiotherapy prior to entering this study. Patients may not be using or anticipate using these treatments after the observed progression or within the time period stated below

• Cytotoxic chemotherapy: must not have received within 2 weeks of entry onto this study (4 weeks if prior nitrosourea)
• Small molecule tyrosine kinase inhibitors (e.g., sorafenib, pazopanib, imatinib), rapalogs (e.g., temsirolimus, everolimus, sirolimus) or anti estrogen therapy (e.g., tamoxifen): may not have received within 28 days prior to the first dose of study treatment
• Antibodies: >= 21 days must have elapsed from infusion of last dose of antibody, and toxicity related to prior antibody therapy must be recovered to grade =< 1
• Biologic (anti-neoplastic agent): at least 7 days since the completion of therapy with a biologic agent
• Local regional tumor directed therapy, including, but not limited to small port radiation therapy (RT), radiofrequency ablation, cryotherapy, surgery: at least 2 weeks since these therapies and all toxicity must have resolved to grade =< 1. If prior craniospinal RT or if >= 50% radiation of pelvis then >= 6 months must have elapsed. If other substantial bone marrow (BM) radiation, then >= 6 weeks must have elapsed
• Stem cell transplant (SCT): No evidence of active graft versus (vs.) host disease. For allogeneic SCT, >= 6 months must have elapsed
• No prior gamma-secretase, Notch or beta-catenin inhibitor
• Investigational drugs: must not have received investigational drug within 4 weeks of study entry, and all toxicities related to prior therapy must be resolved to grade =< 1 or baseline

Concomitant Medication Restrictions

• Growth factor(s): must not have received within 1 week of entry onto this study
• Patients who are currently receiving drugs that are strong inducers or moderate or strong inhibitors of CYP3A4 are not eligible. Strong inducers or moderate or strong inhibitors of CYP3A4 are not allowed from 14 days prior to enrollment to the end of protocol therapy. Note: CYP3A4 inducing anti-epileptic drugs on a stable dose, are allowed
• Must not be receiving non-steroidal anti-inflammatory drugs (NSAIDs) as treatment for desmoid tumor after the observed progression and patient agrees to not use NSAIDs while on study. Occasional use (defined as =< 3 times per week) for treatment of pain is permitted

Peripheral absolute neutrophil count (ANC) >= 1000/uL (within 7 days prior to enrollment)

Platelet count >= 100,000/uL (transfusion independent) (within 7 days prior to enrollment)

Hemoglobin >= 9.0 g/dL (may receive red blood cell \[RBC\] transfusions) (within 7 days prior to enrollment)

Creatinine clearance or radioisotope glomerular filtration rate (GFR) >= 70 mL/min/1.73 m^2 or a serum creatinine based on age/gender as follows (within 7 days prior to enrollment):

• Age: Maximum serum creatinine (mg/dL)
• Age: 1 to < 2 years; Maximum serum creatinine (mg/dL): 0.6 (male and female)
• Age: 2 to < 6 years; Maximum serum creatinine (mg/dL): 0.8 (male and female)
• Age: 6 to < 10 years; Maximum serum creatinine (mg/dL): 1 (male and female)
• Age: 10 to < 13 years; Maximum serum creatinine (mg/dL): 1.2 (male and female)
• Age: 13 to < 16 years; Maximum serum creatinine (mg/dL): 1.5 (male); 1.4 (female)
• Age: >= 16 years; Maximum serum creatinine (mg/dL): 1.7 (male); 1.4 (female)

Adequate liver function defined as:

Total bilirubin =< 1.5 x upper limit of normal (ULN) for age (unless secondary to previously diagnosed Gilbert's syndrome) (within 7 days prior to enrollment)

Serum glutamate pyruvate transaminase (SGPT) (alanine aminotransferase \[ALT\]) =< 135 U/L

• Note: For the purpose of this study, the ULN for SGPT (ALT) has been set to the value of 45 U/L (within 7 days prior to enrollment)

Adequate cardiac function defined as:

• Corrected QT (QTc) interval < 470 ms
• No history of congenital or acquired prolonged QTc syndrome
• No history of clinically significant cardiac arrhythmias, congestive heart failure, stroke or myocardial infarction within 6 months prior to study entry

All patients and/or their parents or legal guardians must sign a written informed consent

All institutional, Food and Drug Administration (FDA), and National Cancer Institute (NCI) requirements for human studies must be met