I don't actually hold spots either. A spot is open when it is open. I do take the deposit if I am filling a spot that is opening up down the road as sometimes you know well in advance when the spot will open. If I have a spot open Sept 1st and someone wants the spot for Oct 1 then they need to begin paying Sept 1 for the spot.
as for leaving, I require 3 weeks notice for them to leave. I use their deposit towards 1st week of care so I don't have to apply any money at the end. Never had an issue that way.