First off, pediatric cancer is NOT the leading cause of death for those 15 and under. It's up there, but "unintentional injury" takes twice as many lives.
Second of all, not many children die (1200 per year), so even if something is the leading cause of death, it doesn't mean it happens very often. Cancer kills 10 times as many people between the ages of 35-44. If you add up all adult cancers it's probably 20-30x the number of deaths.
Also, just because money goes to adult cancer, doesn't mean it can't help. Most drug are developed for adults first and then tested in children. It's not like people are ignoring pediatric cancer.
Before you get so upset you should really sit down and think these things through.
It was also be unethical to develop drugs for children without first testing them on adults, since children can't give informed consent.