Why enable people like this?

Tell them all that you don't expect to have to beg for your pay, They know it is due and they know how much. If fees aren't paid when due, then they will not be allowed to drop off their kid until up to date. Charge fees in advance so if they are late, you have already been paid for what is done.

Rather than squabble and send these notes and reminders, just end their ability to pay late.

If fees are not in on time, send an immediate reminder that it's not been received, that fees + late fee are now due, and that payment is now required by EMT so it can be directly deposited to cover your household bills. Remind them that they will not be able to drop off their child until the debt is paid.