I assume your kids (so are mine) are the smartest little monsters in the world. The problem comes from techie kids who are determined to 'break/hack' things. Armed with the internet, nothing will be truly effective against them. The reason is pretty simple, the internet was never fundamentally designed to be secure... and kids are the best hackers.
This is why I think the monitoring/alarm features in firewalla is going to be much more useful. For example, Firewalla can monitor and alarm if kids are playing games. To a certain degree, Firewalla can also predict/guess how long the gaming session was.
One of our beta mom's told me that her 14 and 7-year-old start to question why there is this orange box next to their router ... (after experiencing problems with the internet).
Personally, if we get enough funding, I really want to create something to help kids learn more about networking and security... Hopefully, we get to that point.