One of my parents is constantly late picking up her child. I have a late pickup fee but I have a hard time collecting this. What can I do?

It’s your responsibility to enforce your rules or not. If you want to enforce your rules, tell the parent you will start enforcing late pickup fees going forward. Include these fees when the next payment is due. Tell the parent that if she doesn’t pay these fees you will terminate care. If you aren't willing to do this, take the late pickup fee out of your contract.