I would try something like this, if you want to go the most professional, polite, diplomatic route:

"Dear [Parent],

Unfortunately I cannot meet you during that time, as after school, I have to pick up my children from daycare. I'm sure you can understand, as a parent yourself, that I have important parental obligations and duties. If you want to meet after school, I will only be available for X minutes, and then I will be obliged to leave. If that does not work for you, we make a plan to meet at a different times. Time that work for me are: before school, during my lunch at [time], or my prep period at [time]. Thank you in advance for your flexibility."

DO NOT say something like "well my contract hours end at 3 o'clock and I don't have to meet with you." As true as that it, that's just going to piss them off and parents going to think you don't care about them or their kid.