First, there is a real deficit of research when it comes to children specifically. We know we have to take a developmental approach and not try to treat kids like little adults, or even like miniature teenagers.
Right now a child as young as six or seven who shows up with an eating disorder can end up being in an in-patient unit where they are treated with 17-year-olds. This is really problematic. Most of us don't want our seven-year-olds hanging out with teenagers who may be talking about all kinds of things that are beyond a seven-year-old's ability to process. Working with their parents and helping their parents problem solve around how to get their child to eat again, supporting the parents in family therapy seems to work, although we don't have adequate research.
Again, we have evidence-based treatments that work for adolescents. We have some research to support this, but again, not as much as we would like. It is very different from treating adults who are independent and who don't have the same kind of relationships with their parents.
To answer your question about needing a variety of treatments, the problem is that even for an adult with other kinds of disorders, we have a variety of treatment options. With kidney failure, you might have a kidney transplant; you might have dialysis. For depression, there are all kinds of different therapy. Somehow with eating disorders, we've decided that if there is one program in the city, that's enough, and if you don't respond to that treatment it's too bad. Your only choice is to try again or to stay sick.