Girlsense.com is good for any age, though as of July 18th, 2012. Girlsense shut down, the only other websites i have even heard of for fashion designing is FPGirl.com, but i don't think they have as much freelance designing.
Dressupwho.com/fashion
Clueless
a good football website 4 kids is the AFL website.
There is coolmath kids can learn things on that educational website
starfall.com is a good website for kids under the age of 7.
If by "kids" you mean people under the age of 18 then of course. In fact many designers became interested and started sketching/sewing when they were teens/preteens.
Future fashion designers academy in Lake Norman,area of North Carolna is a fun place for kids and tweens. They have summer camps and classes year round. They teach sewing and fashion design.
Kids world .com
Webkinz.com is good for kids but you have to buy it first. and free realms and evony
studentsoftheworld is a really good site for kids to have penpals.
Familyfun.com
primarygames.com