I agree with secondtimearound. send a text tonight saying that payment is due in the morning. If they come with no cheque tell them that they cannot leave their child unless they have payment. I know we all hate chasing money but it sounds to me like they are going to see if they can get away without paying. There is nothing wrong with a reminder email and if they still show up without the money then time to get tough and say sorry no money no care. They are the ones putting you in this position and you need to stand up for yourself.
Next time never exchange a cheque until they have the new one in hand.