I think it was mentioned above, but you can block the necessary sites at the router settings. Also, you can set the timer to disconnect required devices from the WI-FI network. Check for applications on the internet. Kaspersky Safe Kids is a good example for you. Also, think about setting the proper parental app for your kid's phones. I met the scammer attack last week in an interesting way, and those guys have been calling my kid and saying that they are my good friends and want to make a draw, so they need my credit card number and CVC code. Hopefully, my boy approached me, and via phone reverse service(i will put the
link here) I acknowledged that those guys scam people.