If I were you to avoid anything like this happening again in the future with any of your clients for any reason, I would amend your contract so that payment is always made at the beginning of each week in advance. I ask for 3 months of post dated cheques each dated for the Monday either weekly or biweekly even if their first day of care isn't a Tuesday. I explain that I need to be sure that my income is regular so I can budget and that by providing me with post dated cheques it means their is one less thing for the parents to remember in the morning. That way if parents forget then they can not leave their child as they haven't paid for their care. I always shake my head at providers who do not require payment in advance. When I read through forums about people having issues with payment it's always those who DO NOT take payment in advance which would have avoided this being an issue in the first place.

I think 24 hrs notice to get payment to you in this case without any kind of penalty is more than kind of you. After that a $5 a day charge if this is your policy should be in effect. Whether mum or dad normally pays is irrelevant. You should have been paid and they deserve the penalty if that's what's going to teach them to communicate better before one goes on vacation without telling the other what's fully going on. Amend your contract, send it home for everyone to sign and agree to with a brief explanation why you are making the changes and say goodbye to this problem.