It costs big pharmaceutical companies over 4 billion dollars to develop and bring a new drug to market. This means that most companies won't even consider developing drugs that won't sell millions of doses a year, in order to make a profit.
That leaves many diseases 'orphans', defined by the Orphan Drug Act of 1983 as a disease "for which there is no reasonable expectation that the cost of developing and making available in the United States a drug for such disease or condition will [be] recovered from sales in the United States of such drug". All childhood cancers fall in this category, being diagnosed in less than 200,000 people in the US annually (the definition of a "rare" disease). But, as has been discussed, childhood cancer isn't all that rare. However, it's still too rare for drug companies to pour money into. This is why, in the last 25 years, only 1 new pediatric cancer drug has been developed.
Dr. Jim Olsen of Seattle has come up with a way to change this method of devloping drugs. Already, his project has developed Tumor Paint, which allows real time, direct visualization of tumor cells. It is sensitive enough to detect mobile cell in lymph ducts, moving between nodes in small groups. It allows surgeons to get clean margins, and to spot small tumors that might not have been obvious in traditional scans. With his organization, Project Violet, donors can adopt a drug that is being studied. These drugs are all derived from the natural defence systems of plants and animals. Tumor Paint, in fact, is from a protien in a scorpion.
While I can't get the TEDxSeattle Video to post directly into this page, it is definitely worth watching at the link above. Dr. Olsen explains how Project Violet works, as well as the other great projects his team is working on, and how his team developed Tumor paint for less than 20 million, .5% of what big pharmaceutical companies would spend.