Since they are a bit new, may have a possible language barrier and have not bounced a cheque, then I'd likely be a bit forgiving and not charge the late fees immediately. I would send out an email and give them 24hrs to drop off the cheque. I'd make it known that you prefer post-dated cheques to prevent this problem in the future. Tell them that you have cheques from the other families that you need to cash and making a second trip to the bank for their cheque is an inconvenience. They are likely just not thinking about things from your end.

If your contract with them states they needed to give post dated cheques then you can enforce that or simply move your pay dates to days they are present (the last day they attend before your usual paydate) so that they are giving it to you before the other cheques are cashed.

When I was full here all families paid in advance and gave me months worth of post dated cheques. I did have one family that for whatever reason was hesitant to give me a stack of cheques. They simply sent their cheque 2 days before it was actually due (it was always for the following 2 weeks of care). This is the family I still have here and I know it is by no means a trust issue they just prefer to give biweekly payments. Since they always send it early it is never a problem for me as if they ever forgot to send it I would have the next day to remind them and still have it on time. If they had ever forgotten I would have given them a second chance before then insisting I get posted dated cheques. They've never been late. They are more comfortable with this method, it is still respectful, timely and doesn't complicate things on my end so there is no problem with not having a stack of post-dated cheques.

I'd never take that much cash though. I have no use for cash in day to day life and would not be comforable with that much cash in the house or on me for a bank run!