I have had parents arrive and change their child at the front door before leaving as they were on their way to whatever activity but it is them doing the changing and I have said my goodbyes and gone back with the other kids by that point.

The preschool issue is no different than if a child went to JK which is something you will have to think about in terms of are you going to offer that service or release kids when they reach that age which is what I do.