It's a very cute wallpaper, fitting perfectly for the little Nia.
You managed to find a way to stay simple, yet not empty. Congrat!
And some critique: you should try to remove some butterflys from the left and right upper corner and both sides so it will have the effect as if the butterflys are escaping/flying away from behind Nia. That's it. Anyways a very cute wall, and nice idea!
Keep up!

I'm also a fan of simple backgrounds. I think the background you've designed so far does fit well with Nia's character, and while I didn't see the theme you've chosen at first, it does make sense after you told us. It could just be because I don't usually look for themes in wallpapers, but you may want to consider what raku said above. I suppose the only real problem I see is that the scan you used isn't quite as sharp as the rest of the wallpaper. I'm not sure if theres a vector out there that you could use of the same image, or perhaps you could vector one yourself, but in my opinion, as it currently stands, the image of Nia stands out a bit. Decent job cutting her out though, I know that can be a pain. Only after looking really close do I see a few spots where something small got cut off, but that's no biggie really.

Oh, and I like the outline work you did on her hair. That looks really good.

Overall you've got something good there (I think). I'm not sure how feasible it is to get a sharper image of Nia, but you may be able to work something out, or you could just simply leave it as is. I really like Nia as a character, though, for me, this wallpaper is a little too pink for me to download, but I'm guessing that doesn't bother you, and it probably won't bother most of the other people around here.

Props and keep it up!

Its look cute, for the girls of course, nice job dude