Same here. I really like Firefox for that reason. I don't know how it works, but it seems to work a lot better than some of the others. Limiting my kid's experience online is unapologetically censorship on my part, but then I'm not the government & my home is not a democracy. I refer to it as a "benign dictatorship" :)
Firefox isn’t perfect though, and you can still end up going down a blind corner with popup blocking enabled.
Personally, I purchased a router with filtering software built in. That way, no matter who connects to the internet gets filtered—including devices that don’t have filtering capabilities like our Nintendo Wii. It actually contacts a central server first and if it is greenlighted, then the web transfer goes through.
The biggest benefit is that it protects from unscrupulous sites that install spyware or malware without warning. Just in case there is a real need to look at what’s behind the block page though, there is always a password parents can type in to unblock just once or for a set amount of time.