There is an interesting research article created by the Neilson Co. at Adweek. In short apparently websites geared to children such as Webkinz World jave a little to no ads. However the older the demographic, the more web ads are shown to that group. Sites geared toward seniors have the most ads.
Rest assured when your kids play webkinz the website has few ads, just like this site.
www.ewebkinzstore.com
Click Here to see the Article