I am a recent convert and now major devotee of the "rules" on Teri's Tightly Curly Hair Site. She, your daughter and I are all in the same hair boat, and you can see how long and healthy her hair has grown by following her rules which, in a nutshell are: be careful of your products, never ever use a comb, always brush with a good brush, never brush dry, brush/detangle with conditioner and never let the hair dry without conditioner left in it.

As for straightening, anything you do with heat, whether it's to dry, to straighten or to curl, is going to cause damage. The best you can do is to lessen the damage, but you can't make it "safe." Despite her wishes to sometimes go straight, I think making sure her hair is as healthy and undamaged as is possible (trim her split/dead ends!) for a few months before getting close to her with a heated implement is a good idea. The nice thing is that you have the summer to do that, skip the hair dryer while it's warm and that'll put her well on the road to healthier hair. (And when her hair is soft and her curls move and aren't frizzy or tangled up, she might not want to go straight as often, too.)