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Against casual attempts, yes, it's not that hard. But someone dedicated enough can create their own DoH forwarder for free on Cloudflare (the subdomain will be under .workers.dev, shared by all CF Workers), there are other methods but they require cards and more technical proficiency than clicking around, which at that point it's easier to just set up one's own VPN server, skipping any DNS blocking.
To inject a little paranoia, DoH spec and implementation don't actually require the providers to only use /dns-query, it's possible (and very simple) to create an innocuous-looking website with /supersecretdns serving DoH, or directly on the homepage itself (the request for DoH vs regular webpage has different header), but if your kids are already that proficient, no way to stop them aside from plugging off the router.
I run Pfsense and am able to block most common DoH services. I’m sure you will be able to configure similar options on opnsense. The best way to do this is a DNS block through AGH and an IP block with opnsense. Firefox provides what domains to block to disable their DoH, https://support.mozilla.org/en-US/kb/configuring-networks-disable-dns-over-https. You can also add these two lists to block most other common DoH services, https://github.com/oneoffdallas/dohservers, https://github.com/Sekhan/TheGreatWall. These lists will work with AGH for DNS blocking and for IP blocking aliases. If you have any Apple devices on your network you can use these domains to block private relay, https://raw.githubusercontent.com/Rogacz/private-relay/main/pr2.txt. I recommend you add these private relay domains as a custom entry in AGH to return NXDOMAIN so that the device shows that private relay is unavailable versus using a NULL response where it will say it’s available when it really isn’t. With these lists added to DNS blocklists as well as IP blocklists I have seen almost no DoH services getting through. The only service that I’ve experienced getting through the rules so far is Next DNS since it uses different IPs depending on what is fastest for your location, making it harder to block. I found a way to discover the IPs for their servers near you and will edit the post if I find the instructions again. Also make sure to completely block port 853 to block DoT. Lastly using these instructions from Pfsense, you can redirect or block all DNS queries that aren’t destined for your AGH instance. The instructions should be transferable to opnsense.
I run Pfsense and am able to block most common DoH services. I’m sure you will be able to configure similar options on opnsense. The best way to do this is a DNS block through AGH and an IP block with opnsense. Firefox provides what domains to block to disable their DoH, https://support.mozilla.org/en-US/kb/configuring-networks-disable-dns-over-https. You can also add these two lists to block most other common DoH services, https://github.com/oneoffdallas/dohservers, https://github.com/Sekhan/TheGreatWall. These lists will work with AGH for DNS blocking and for IP blocking aliases. If you have any Apple devices on your network you can use these domains to block private relay, https://raw.githubusercontent.com/Rogacz/private-relay/main/pr2.txt. I recommend you add these private relay domains as a custom entry in AGH to return NXDOMAIN so that the device shows that private relay is unavailable versus using a NULL response where it will say it’s available when it really isn’t. With these lists added to DNS blocklists as well as IP blocklists I have seen almost no DoH services getting through. The only service that I’ve experienced getting through the rules so far is Next DNS since it uses different IPs depending on what is fastest for your location, making it harder to block. I found a way to discover the IPs for their servers near you and will edit the post if I find the instructions again. Also make sure to completely block port 853 to block DoT. Lastly using these instructions from Pfsense, you can redirect or block all DNS queries that aren’t destined for your AGH instance. The instructions should be transferable to opnsense.