Originally Posted by Blackcat31:
I would simply attatch a note to their next invoice stating "Oops, it must have been an oversight or a mis-understanding but my rate is: " and explain what they should have paid.
I would add the missing amount to their bill and go from there. If you don't fix it now, you will always have issues and I personally see nothing wrong with pointing out an error. We are all human, it happens. Correct them now and you won't be resentful and won't have to correct them in the future. 
That's what I'm gonna do at pickup. I also have their full time contract ready to go home today as I didn't have it ready at the start for last week. This full time thing was just established so everyone is all confused as to what's going on. My schedule is all wacky this year:
One dck went from 3days to 5 days (which was new and was not informed about till day back)
One dck went from 3days to 2 days (which was new and was not informed about till week back)
One dck went to full preschool
One dck went from 2-3 days/wk to maybe one.
Everyone last year was mon, wed or fri or at least 2-3 days a week. Since that was the case I changed to be mon, wed, fri exclusively this year.
So far I have none of that. I went a whole two years with mon, wed and fri kids without having anyone want full time. NOW I change it and everyone is all OMG SO CONFUSED. I did that to mainly secure the fact I'd only conti use to have them mon, wed and fri as I want to get my own son into extra things like ice skating/hockey and swim soon.
So now it's all kinds of clustered and I'm not too happy. If I can have two full timers I'll drop the ver limited part time kids so I've offered a discount full rate as an incentive.
So far, big bites in my butt all around.