In that case, I agree with the others and say leave this up to the router - not only is it far more easier to set up, it gives you/your kid the freedom to switch between distros/OSes, and you can even swap computers without worrying about having up the controls all over again.
A friend of mine was in the same situation as you (he’s also a Linux nerd), and he ended up with the router thing, and after extensive research, he decided to get a Synology router as it had all the features he was after (mainly limiting access times, monitoring and reporting). See: www.synology.com/…/device_content_control