Well going off what my sisters have shown interest in over the years, heres a few that haven't been mentioned.
Crash Bandicoot 2. Simple mechanics, doesn't have too much of a twitch factor. Oh and it's available on the PSN for 6 bucks.
Chocobo Racing. Nowheres near as good a game as Mario kart or CTR, however its a fair bit easier, has a cutesy feel good plot, and most of the characters are freaking adorable. You'd need a PS1/2 and it would probably cost you 30-40 bucks however.
Kingdom Hearts. Simple RPG game-play, not too difficult, the Gummie ship and freaking Disney characters. 2 may be a better option then 1 if shes got a shorter attention span. The cut-scenes are unskipable and some of the earlier ones are really really long, which they fixed in 2.
Oblivion. Stick it on easy, let her do what the hell she wants. Almost impossible to die on easy mode, it's not a bad way for her to get used to moving a character in 3d space and she'll most likely find some activity in it that she enjoys.
Nintendogs. Do I really need to explain this one?
Harvest Moon: More Friends of Mineral Town. A well done social aspect to it, again fairly simple to pick up and it keeps throwing new things at you to do the whole way through. You can fail I guess, however doing so simply means you can't keep playing past the finish and it takes a good while to get to the finish anyways. Harvest Moon: Friends of Mineral Town, or Harvest Moon: Back to Nature would also work. All basically the same game, however M/FoMT is an enhanced version of Back to Nature, and More Friends has you playing as a female character. Back to nature is also likely to set you back 80-90 bucks minimum these days.
Yea not all of em are co-op (Infact only Chocobo Racing is.) but as said I hadn't seen any of them suggested before yet so.