Great advice here!!!
I've had a similar situation - the parents of two kids never seem happy (the kids love it here), about a complaint a day, trying to get reductions in fees, were late to pickup until I enforced my late fee for it, don't pay unless I ask for it, nasty comments regularly. I'm looking for replacements so that I can terminate the contract. Too many issues. Life is too short to deal with disrespectful ----.
It's been a real lesson on becoming more assertive. I taught elementary for several years and never saw these kind of boundary/respect issues. Yikes!!!
You do hard and important work caring for kids - Keep up the good work, y'all!