The New Yoshi's Island on 3DS may be a dissapointment if you played the original, but it's pretty easy, cute and fun.
Super Mario 3D Land on the 3DS again, I know it's Mario, but it's also accesible and very good, much better than Yoshis Island.

POKEMON! :)

Maybe Pokemon X or Y?

Also you should check out trailers for the new Kirby game that was just released (Kirby Triple Deluxe).

I have it. It's almost perfect for younger kids. The only thing is it deals with money, and I hope she can read because there's a lot of diologue. 

Disney Magical World would probably be perfect. That would be my #1 suggestion.

There's also:

Kirby Triple Deluxe

Mario Kart 7

Style Savvy Trendsetter (which surprisingly has a 76 on metacritic)

There are already many good suggestions, so I'm just gonna add Luigi's Mansion 2 to the bunch.

The game puts you in the shoes of Luigi and your on your way of catching ghosts. (really cute ones at that) Its features are for example a wide variety of ghosts, all well designed, as well as quick and responsive gameplay and of course the lovely crafted environments, which you can explore.

mario kart

Pokemon or Mario Kart. How does she go with the games on the 2DS? I'm not even sure if it has the same software as the XL though but that could provide some more insight.

Kirby Triple Deluxe. Probably the best kids game out there.
Animal Crossing
Style Savvy
Art Academy
Nintendogs
Disney Magical World
Pokemon X/Y

The game my daughters enjoyed most on 3DS has been Luigi's Mansion. They were 5 years old (twins) at the time, and they basically made the mom play, and they watched and participated that way.

What I found really good about them playing that together, is the kids would then draw things from the game, and make stories, and just enjoy talking about stuff they were experiencing in the game.

My little girl is 13 now and she's a DS and 3DS fanatic. Aside from downloadables, I'm gonna take a quick look at her collection:

Style Savvy Trendsetters
Animal Crossing (aka: Crack!)
A few Scribblenauts games.
A few Cooking Mama/ Gardening Mama/Crafting Mama games
A few Kirby games
Mario Kart DS/7
Monster High
Nintendogs + Cats
Some shit like Cake Mania 2/3, Fast Food Panic, etc.
Wario Ware Toouched and D.I.Y.
Tetris

Some other stuff. And I know she has a lot of digital games because I bought em! Can't remember, though. Just looked at the shelf in her room.