Just go with Brawl.

Ok get galaxy for her. jkkkkkkkkkkkkkkkkk

Unno, mario kart? little kids love that

I have a little sister around the same age of your niece. She loved Melee and played it with me all the time. Heck, she and I played SSE together as well. Good times there.

However, she also loved Super Mario Sunshine and Galaxy, and even though I told her it was too hard for her, she got all the way to Windfall island without my help in Wind Waker, which impressed me. ^___^

So I'd go with Brawl, it's a blast to play with anyone, and a lot of the modes are now Co-op.

Get her metroid prime 3. Samus could be a rolemodel. JK

 My little Sister Loves to play Brawl and she's only 6, but yeah she also loves Mario and Sonic

*Insert random Nintendo superiority remark*

Well, Brawl's a good choice. Cooking Mama might also be good, along with Mario Party. I think her preference in Brawl shows that she likes games that she can pick up and play.

Brawl would be a great present if she enjoyed Melee, but if you choose something else, Rayman Raving Rabbids 2 is also good.

You really can't go wrong here. 

I don't get why people are suggesting shitty party games. Kids aren't stupid... They're perfectly capable of playing great games.

Yeah, but we're talking about a girl here...

 Girl's tend to like problem solving and puzzles.

I'd go for Zelda Wii or Okami. Get her something good to play. She's already going to have to play Wii sports.