When I read the subject line of this post, I thought it was that children weren't welcome at the models.
First, I think you're being a bit dramatic by saying they're not welcome. It seems to me they're just not welcome to take advantage of a playground where they're not residents (yet). I have no problem with that.
I do wish that builders would ban children, though, from their models. Most kids are utterly bored with model home viewing and are dragged along by their parents. Some kids are well-behaved (mostly absorbed by their iPads or whatever is playing on the TV). Others run around from room to room screaming, jump on beds, etc and are just generally nuisances. Certainly, this is the fault of the parents for dragging them along and for not teaching them proper behavior in such settings. I'm sure that children cause a lot of damage to model homes. Frankly, my dog is better-behaved than many children, yet most model homes won't allow my dog in (except for TNHC, where my dog is always welcome).