I tried to retrace my steps as to how I stumbled upon this site. I was exploring (23 Things) short list on the Web 2.0 awards list, and somehow I found it. Nevertheless, kneebouncers.com is one of those "free" games-for-your-children-to-play websites that doesn't cost you any money. It looks good at first, but the minute you complete their 'registra-tion' (but, seeing through the thin veneer, you'll know that it is nothing more than an advertiser's survey for sending you unwanted ads), and you'll be sorry!
There's really nothing FREE in this life--everything has strings of some sort! This will cost you aggravation, in the long run, by wading through tons of the SPAM that is attached.
Considering this, it would be mentally cheaper to physically take your children to a arcade for a day and let them have a blast, rather than, having to later, blast your computer because of all the junk mail you'd get!