I think that it's really up to the parents to let you know if they have something they would like improved etc about your daycare. Like the previous poster said they might come back with a laundry list of demands and then what?

I think it's safe to assume that if they are still with you and don't complain too much then there is nothing to worry about.

If you need them as a reference, then just ask them flat out if they would be ok with that.