Wont work if they are using a browser with DNS over HTTPS though.
Might be more effective with phone apps, if they aren't crafty and use their own DNS (eg I believe Googles own apps bypass your routers issued DNS and use Googles own directly).
Ultimately DNS blocks are far less effective these days as we move to technologies designs to get around DNS blocks, for security reasons (not allowing bad ISPs to hijack DNS).
Its one thing I do on pfSense, I hijack all traffic aimed at port 53 and force it to go via Unbound so I can do my own filtering. But again, DNS over HTTPS in the browser will bypass that.