My daughters are a older than your daughter (8 and 9) and they love the game, especially Anna and Elsa; I'd say they use those two characters 75% of the time when they play. They haven't really built their own worlds in the Toy Box feature that much (similar to Little Big Planet) but they've downloaded other people's Toy Boxes as well.
As far as the console, I've heard (unofficially, of course) anecdotely on other forums that the PS3 version is the best version to get. Once you get into this though, be prepared to drop a lot of moolah, especially if you want to get all the characters. Luckily I was able to get eight characters through an Amazon BOGO sale (should have bought all the ones taht were offerred...D'OH!), and Target has a Buy Two Get One free sale this week, allowing me to complete the collection (with the exception of the Cars set). I'm not even going to attempt to get a full set of the Power Discs.
Hopefully, Avanlanche and Disney will release more playsets (especially Frozen) for the inevitable Disney Infinity 2...as well as making it compatible with the original game.