Not everyone likes kids,same as not everyone likes pets. I've been places where there has been a heap of kids, running through sites, making general kid noise, which is fine for the parents of this kids, because they have said, go on go play down the rd with the other kids, so while mum and dad have a nice peaceful camp, that group of now 15 kids is running around the whole park disturbing everyone.
Maybe the owners of the park got sick of having to fix things that kids destroy or vandalise? Who knows.
Ps I like kids and it doesn't bother me, just pointing out another view.
Edit, when we went to Vanuatu a few years ago, we stayed at irriki island, one of the mains reasons for this, no kids allowed. It was great sitting by the pool, without listening to kids yelling and screaming, or bombing in the water every 30sec.