“Many insurance companies will not pay for an expensive drug...

“Many insurance companies will not pay for an expensive drug that's used in a way that's not listed in the approved drug label.”

That doesn't just apply to cancer treatments I suggest it is probably across the board.  In relation to pediatrics, an article from Boston Childrens Hospital Division of General Pediatrics, What to consider when considering "off-label" drug prescriptions for children includes these words;

"While the rate can vary greatly depending on the specialty and environment, in general, less than 50 percent of medication labels have any guidance for providers on a drug's use in children," says Florence Bourgeois, MD, MPH, an emergency physician at Boston Children's Hospital who studies pediatric drug development practices. "At an academic medical center, that rate might even be as high as 60 to 80 percent for the medications used among hospitalized patients."

Further into the article Bourgeois is mentioned again;

She notes that providers are sometimes reluctant to prescribe medications to children off-label for fear of causing harm. And while that could prevent children from receiving a potentially beneficial treatment, some hesitation is understandable. For instance, there can be greater medical and legal risks when writing an off-label prescription. Also, some insurance carriers will not cover the cost of drugs prescribed off-label or will discourage patients from filling the prescription.