Personally, I'd use the router to limit access to locations and times. It's more reliable, easier to do, and lets you be less picky with your distro.
Using a DNS level content blocker like Adblock DNS is a great option, IMHO, and is super easy to setup.
(For the record, parent of 8 and 11 yr olds)
Having your router limit internet connectivity time is effectively the same thing these days. There are some things they can do offline but not much anymore.