I have a cracked vertebrae in my c2/3 from a bomb proof broke horse. There is a most likely a reason someone is selling a horse, the only good reasons is the death of the owner, divorce, financial hardship. It would be like me selling a top notch hound.This is very dangerous advice unless you have someone with the experience to show them how.
Kids need a bomb proof horse to show them the ropes. There is a reason old horses are known as babysitters.
I personally believe in the op situation just doing lessons on a variety of broke lesson horses is way more beneficial and less expensive/work and his daughter will get a wide range of horses/experience. I may have stated my response incorrectly previously. I wasn't indicating that they should buy a young horse, bc they don't have the place or time to put into it.