Obviously I'm not a parent so don't take my opinion too seriously... but I'd say, so long as you mix the paints for her (you don't want her butchering your models), give her something simple (like a Marine) and try to guide her hand, maybe give her an example moddel (so she sees that, for instance, the exposed parts of the armour are supposed to be black padding), you should do fine.
I also suggest that rather than letting her try to do details... you get her to follow standard guides but stop on like the third step, thataway she can salvage the models when she's older.
So, using watered down paints after it's been undercoated, get her to apply the basecoats to the model and the weapon, maybe let her try to do the eyes.. then get her to leave it.
I know if someone had gotten me to do that when I was 7, my models wouldn't be sitting in brake fluid right now