Welcome to the forum, first off! and welcome to the daycare field!
You are totally right, very wise for just starting, takes a lot of people many mistakes before they know what to do and not to do! what you need to do, is not allow them to change their days, They WILL continue to do it IF you let them. and if you are willing to take the child a dif day, let them know there will be a days extra charge, due to the fact that that is not a regular schedules day for their child....this way they either pay you extra, which helps make it worth it to change the schedule IF you have the room or desire to do so....or they wont ask to change the schedule around due to the fact they dont want to pay extra. I have learned the more you let parents push, the more they feel entitled to!