Well it's like employers that seek technical solutions to administrative issues. Ineffective and only makes sense for CYA liability.
If your kids are going on dating sites and dating unacceptable people or whatever - well how is that different than real life? Or as another poster wrote, about viewing beheadings. If my kids decide they want to watch such things, I suppose they'll find a way. I'd be more concerned why they wanted to watch it; isn't that the real problem, not the technical means on how they accomplish it? (Well I suppose beyond once out of curiosity.) I'd be just as upset to find them torturing small animals, but I'm not gonna hide scissors and knives.
It doesn't seem plausible that any kids are going to decide "Hey I wanna hook up with people online, but I got an error. Oh well. I'll play with LEGO instead."
And hell, if my kids read Stormfront or RedPill and decide "hey, that makes sense", then I've failed so hard at teaching them how to evaluate probabilities. At that point, DNS would be the last problem on my mind.
Accidental viewing, eh, I'm not too concerned. When my daughter was learning to spell, she was searching for monsters. Unfortunately, her phonetic attempt came out "man store", which was some bondage or leather thing. We had a good laugh and moved on. Maybe I'm wrong and something will trouble them for life, but it just doesn't seem likely to the point of spending any effort or worry on it.
Do you really think accidentally viewing something unsavory is gonna be a big deal?