Run pihole with a MikroTik router at your houses demarc.

Set up firewall rules in the tik to redirect any packet with dst tcp/udp 53 outbound on wan to the pihole. If you’re worried about dns when pihole is down, create disabled rules to allow the dns traffic as well, then set up a scheduler script using if blocks to toggle the sets of rules depending on status. This will force any client on your network, even one with hardcoded dns IPs, to use the pihole.

If the client is configured for DoH you’ll just have to build out an ip block list in the MikroTik and block all known DoH IPs.

Create firewall rules to drop all vpn traffic you can, combining port based rules and similar IP lists as for DoH, but subbing in VPN provider IPs. If you want to route your home traffic over a vpn, set up a vpn client in the router itself and basically site-to-site your home to the vpn provider you use.

This would block the vast majority of kids trying to bypass parents blocks, but it’s asking quite a lot from the parents who don’t have experience configuring this stuff already.