The blue lake is not too big for her when up and riding for learning the basics, but yes too big for anything else. If she is one of those that has trouble popping a big board up/sinking it some (leg strength) then it will be a struggle and negative for learning how to pop up. This is a 50/50. Some smaller people have trouble with big boards others do not. She will be better off learning on a board you get for your youth. You should decide if you want a surf board or a skim board.
People learn on blue lakes and more so broadcasts because of availability. It is what the shops have and sell to the customers, it has nothing to do with them being good boards versus bad boards or good learning boards, etc. If you do not have a problem dropping a 1K for boards you will out grow in a few months, that is cool. You could also get two nice customs for 1K, which will allow you to progress more.
Other than that if you need help setting up the wave for the X10, I and many other can help. The X10 throws one of the best smaller boat waves out there. Similar if not as good as an RZ1.