setting up a laptop for use with my kids, 7 year old boy and 6 year old girl
boy, did you time it perfectly..... 
follow the link to plenty of Plastic Fantastic accessories......
(links have been acting up recently, may have to copy/paste to a new browser page)
https://www.wincustomize.com/skins.aspx?search=Plastic%20Fantastic
there are some screenies there also to get some ideas from....
Vlad has some excellent walls kids would love as well.....lower resolution ones are free....
http://www.vladstudio.com/wallpapers/
if you're using XP bootskins...there is a koala pirate and Bart Simpson boot...
http://sydneysiders.wincustomize.com/skins.aspx?skinid=10032&libid=32
http://sydneysiders.wincustomize.com/skins.aspx?skinid=10043&libid=32
and some bright & colourful simple scenes...
http://sydneysiders.wincustomize.com/skins.aspx?skinid=10107&libid=32
http://sydneysiders.wincustomize.com/skins.aspx?skinid=10037&libid=32
http://sydneysiders.wincustomize.com/skins.aspx?skinid=9974&libid=32
http://sydneysiders.wincustomize.com/skins.aspx?skinid=9968&libid=32
and lastly...this site has some lovely Disney kids wallpapers and others....
http://www.wallpapervillage.com/category.php?pagenum=0&catid=28
Edit..
yes, you will have to copy/paste into new browser....seems all links are up the creek....
*sigh*...