Using DNS for parental controls is likely not very effective except for very young children. Most devices that you would give to children (anything from Apple, most Android devices, Chromebooks, Windows) offer parental controls that aren't as easy to circumvent and work on networks other than your home network. Of course, it might be good to use both as a sort of defense in depth.
Parental controls on chromebooks are awful. Family Link itself is a badly put together product, but its claim to support ChromeOS is woeful, half of the stuff doesn't work, and mysteriously disappears from the app when the device in question is a chromebook.
They haven't done a good job. FWIW I work for Google, mean no offense to the team that works on this, but it has all sorts of problems, including service reliability. My wife is taking UX design courses and one of her final course projects was a multi page paper breaking down a bunch of our issues with it :-)
We've been down this road for months. Issues at home with mental health damaging content being accessed repeatedly, and I ended up signing up for OpenDNS; problem solved [for now].
It's been pretty effective up to age ~13. I'm not really sure after that, as I don't really bother to monitor.
I've blocked malware too by having DNS controls, presumably that won't be possible with DoH: Chrome (without addons) might check to see if I've set a local DNS provider but ad-/app- embedded malware won't.
