Really there aren't too many choices in the DS world

There are lots of colors of course! But for the DS your main choice is the old heavy one or the newer light one. I would have her go into a game store and try the various demo units out to see which fits into her small hands. That'll be the key thing - which she can hold onto the best.
I'd not get the DSi which is the internet enabled one. I think she's too young for that.