That’s interesting indeed. So in theory, you could put the child as an adult in the booking and they couldn’t denied lodging.
The only rooms that would accommodate 3 in the booking are the Villa and Cottage House. So as long as the parents don’t mind spending an extra $1000 for the child (adding a 4th only adds minimal incremental costs) to book into a Villa, they can bring their children.
I have seen adult, college aged, children at Ventana so this makes sense.
Ultimately the question for parents is why would you want to bring small kids to a place where your goal is to relax and get away from children…. That is probably the biggest reason we don’t see kids there.