Sirolimus, is a potent immunosuppressive drug that is approved for use in prevention against allograft rejection following solid organ transplant. It has anti-tumor effects mainly by blocking signals which drive cells from G1 to S phase during cell cycle through inhibition of mTOR, thus inhibiting cell growth. Sirolimus, as well as other mTOR inhibitors, has shown anti-tumor activity in pediatric solid tumor xenografts. Children with relapsed and/or refractory solid tumors are in need of novel therapeutic approaches. One option for these patients is the use of prolonged exposure to low dose antiangiogenic chemotherapy, with agents such as etoposide and cyclophosphamide. In this phase I trial the feasibility and optimal dosing for daily sirolimus, in combination with daily celecoxib, and low dose etoposide alternating with cyclophosphamide, will be determined in children with relapsed and refractory solid tumors. p70S6 kinase inhibition will be used as a surrogate for mTOR inhibition. The potential immunosuppressive effect of sirolimus administered on this schedule will be assessed by serial lymphocyte subsets and assessment of memory T cell number.