If the parent hosting had come to ALL of the other parents first saying, “Hey, per head it’s going to cost $X, if by a certain date you can’t make it, fine, but after that day if you don’t come I would appreciate a payment as your child will have been accounted for and a spot reserved in the activities.” Or some variation. If they were going to pay it all themselves then DON’T ask the parents for money, period. But if the guests’ parents were aware they’d be paying for their kid that’d be a different story. But, regardless, do NOT tell a child or give a note to a child that their parents owe money. Kids don’t understand that and they feel scared and guilty thinking their parents are in trouble.