GROUP A: Patients receive oral gamma-secretase inhibitor RO4929097 once daily on days 1-3, 8-10, 15-17, and 22-24. Treatment repeats every 28 days for up to 24 courses in the absence of disease progression or unacceptable toxicity. GROUP B: Patients receive oral gamma-secretase inhibitor RO4929097 once daily on days 1-5, 8-12, 15-19, and 22-26. Treatment repeats every 28 days for up to 24 courses in the absence of disease progression or unacceptable toxicity. Patients may also receive concurrent oral dexamethasone twice daily on the days of gamma-secretase inhibitor RO4929097 administration. Once the MTD or recommended phase II dose of RO4929097 plus dexamethasone in children with solid tumors, including CNS tumors, or lymphoma has been identified, this dose is used for patients with relapsed-refractory T-ALL (phase 2 portion of the study) to evaluate RO4929097 in combination with dexamethasone using one of the studied schedules.