That's the million dollar question! Unfortunately, we have so much immediate gratification with cell phones and internet access. Your kids will have to be a little bit willing to listen to ideas- What are their hobbies? Maybe help them get interested in a new one? I have been fortunate because most of the time my daughter monitors her own usage. Good luck, wish I had the answer for you!
I've unplugged the modem and told them that the internet was down. It worked like a charm and we all sat down to do a jigsaw puzzle. It works like a charm and have done it a few times...no harm just bonding.