eek gads.

I think they need a nanny.

But, to answer your question:
I decide when we transition here, not the parents. Unless there is a medical reason (which I can't think of any offhand that would require a child to stay in a crib), they transition here at 2 yrs old. I have done it as early as 17 months with one child who was a dream, but most of the time, I wait until 2 and say that to families at the initial tour.