My son wears them, started aged 12 but I have heard of younger children having them.
His experience has been fantastic. It has completely stopped the deterioration of his eyesight, he loves the freedom of not wearing glasses and he is very responsible about putting them in/out and cleaning them. He has worn them for 18 months.
His eyesight is much worse than your DD, he is approaching that -5 (and you're right you CANT have them at higher prescription levels). But he was "losing" 0.5-0.75 per year. But since the ortho-K it has stabilised - I do need to add that my DH's eye started deteriorating at the age my son's started and also stabilised in his early teen years, so this may not be down to the ortho-K... but he loves them.
Only drawback has been camps for school - there isn't anywhere safe to leave them during the day, and he has to take peroxide solutions etc to clean them, and they need to be kept safe/undisturbed... normal contacts you wear all day and store at night, which is easier to secure. But he has slept without them for 3 nights, and although he could tell his vision was worse, he could still see enough on day 4 of camp to get by. Not sure what we will do for longer trips with no secure place to keep them (cross that bridge when we need to).
And although we are delighted with the ortho-K, my daughter will not entertain the idea of wearing any lenses, night or day. So her vision continues to deteriorate, sigh.