Three other things to consider 1. You are on stand-by in case of emergencies which require you to pick up from pre-school at short notice 2. you charge full fee as unable to give that space to another child 3. Your fee also guarantees the space for holiday care when the child is with you full time as preschools are closed
Don't justify too hard as it comes across apologetic and unsure of yourself. Just factually tell then the terms and conditions. This is what I would expect to find in any other service providers policies.