Early phase trials are crucial in developing new therapies for poor prognosis childhood malignancies. Outcomes and toxicities of children treated on phase I/II trials at the Royal Marsden, one of the largest pediatric oncology early phase trial units in Europe, were examined to provide a baseline dataset and generate hypotheses. All patients recruited over a 10-year period to December 2011 were included. Variables including baseline characteristics, time on study, survival, toxicities, and admissions were collected. Seventy-two patients were recruited to 21 trials (5 phase I, 16 phase II; overall 12 involved molecularly targeted agents). Median age at consent was 12.4 years. Dose-limiting toxicities were rare in phase I trial participants (2 of 15 evaluable patients, 13%); the most common reason for leaving trials was disease progression (76%), rather than drug toxicity (1.7%). Median time on trial was 1.3 months (phase I patients) and 3.3 months (phase II). Early phase trials in children are safe and unexpected toxic side effects are infrequent. Patients and their families are willing to travel to access novel therapies, although the overall prognosis for these individuals is poor. Continued expansion of the portfolio is needed ultimately to improve the outcomes for those with resistant disease.