I'm surprised this topic didn't go in a complete opposite direction...
I say, Kirby Super Star Ultra or New Super Mario Bros. for the DS. Both are great games - and don't underestimate a 6 year old's ability to play games. Reading aside (of which there is none in NSMB or Kirby), if the kid likes the game, she'll step up to the challenge. It'll help if there's someone around often enough (you?) to, perhaps, help beat a level or a boss here or there. You may have to be her own occasional Super Guide.