An interesting situation.
We have kids. They are (in my very subjective opininion) well behaved. We choose however, to avoid camp sites that are visited by lots of families. We prefer the smaller, greener camp sites where (in theory) there is 'nothing to do'. And a certain type of kid would be bored straight away.
Why? It's not that kids can't behave themselves, it's that some can't. Just like (and as a result of) some adults that (should) raise them. And most of the adults who can't behave themselves or those who have kids that can't, prefer sites where there is a lot of activities and entertainment for those kids. So parents feel they have time to themselves, but this sometimes results in nuisance for others. Not a good thing. Also, camp site owners who earn money from families with kids, sometimes tend to allow too much. They should set (reasonable) rules and uphold them.
When I see 'adults only' camp sites I always a get a bit annoyed on the one hand. (adult only sounds like a strange camp site, btw

). Because it so specifically excludes kids, when many kids you will hardly notice and there's a lot of annoying adults as well. But on the other hand I get it, because many camp sites with families/kids are sites I wouldn't want to stay either.
I guess, as with all things, its nuanced. You have well behaved kids and not so well behaved kids. And you have parents who try to keep their kids from being a nuisance to others and parents who don't so much. And then there's adults without kids who can be just fine or not so well behaved. And site owners who try to create a place that is enjoyable for all or who favour one type of guest.
Choose your sites wisely