If you want something more "grown up" but still whimsical, have a look at the Walt Disney Signature wallpapers from York. They are all patterns inspired by Disney artwork from different animated films, but there is nothing childish or theme-ish about these papers! I love them!
http://www.yorkwall.com/STATIC/PRODUCTS/BRANDS/LANDING/TABS/WDSINSPIRATION2
Of course, if you WANT more colourful or youthful images, then you can check these out.....
http://www.yorkwall.com/photo/Children

Thank you so much! I LOVED those wallpapers! I don't think that any of them will be the right shade for the walls, though. But you definitely gave me some great ideas!:)

Thanks! I really like the Pink Toss the Bouquet Wallpaper! We're thinking of that one!

No problem, my pleasure.

I like the cool kids as well!