Using a (regex?) loose heuristic analysis of content on a website, this feature, if enabled, would watch for the 'over 18' query and automatically click to 'not over 18' link.
Seems like this could be an extension. Or even, I know there's uBlock Origin filters that remove cookie banners (not click, but remove them, if that would be an acceptable solution). Either way, good idea. Not trying to bikeshed.
There are nsfw filter lists that you could load on to ublock or this one for hosts https://github.com/columndeeply/hosts which should prevent accidental nsfw viewing.
And talking with your kids so that if they get introduced (by internet or pedo) that they come to you and ducuss it.