I'd type up the invoice, have it e-mailed to them, and have it printed it off to hand to them at drop off. Child wouldn't be able to be checked in without paying the late fee in full.
"Oh, I'm sorry. All account balances must be paid in full prior to admittance. I can take cash or check for the $55.00 late fee from yesterday. Thank you!
"
They'll be miffed. Just stand there and smile. Afterall, YOU weren't late...they were...and YOU missed mass. They should feel poorly about it and apologetically hand you the late fee. If they don't, then let them be the awkward one and you be the polite business owner standing there smiling.
I used to care about awkwardness with the parents. Now I stand and stare like I have no idea it's an awkward situation.
