I don’t know if it’s exactly what you’re looking for, but we use K9 as a blocker. There’s a setting you can enable to bark when the blocker is activated. It is for computers only and is not on the whole network. But it is free, and is more than sufficient for us so far (my oldest is in 2nd grade, and never has unsupervised Internet access.)
One thing you can do on your network is schedule it to turn off overnight (or other times.) That is also free.
