Have a frank chat with mom. "Sally, I am tired of receiving my payment late. I know you have mentioned that you have trouble remembering pay dates but I really need to pay my bills and get supplies for daycare. Going forward, here is what we are going to do." And then lay out your new plan. And stick with it

I send home a reminder bill on Tuesday prior to Friday's payday.

Attach late fees to payment not received by 'due date' and 'due time'. I have mine due at drop off (no later than 9 am) the Friday prior to the upcoming weeks of care. $10 late fee per calendar day starting at 9:01
.
Do not let the kids in for care until all fees (including late fees) are paid.

If she is late x amount of times, tell her that you will need post dated cheques. Even EMTs can be done same day.

Really, you shouldn't have to beg for money.