I think it's good and perhaps these parents will receive a wakeup call that they need to improve their child's behaviour. Especially if they receive the same opinions from the next daycare provider or the next school teacher. Don't these people want to raise polite children who respect others?

As long as this family won't be able to smear your good name or reputation you may as well be honest. But as Alpha said, you may want to be really careful about your wording.