When you host a party, you incur the costs.
If you look at the invoice, the parent hosting stiffed the ski hill, and the invoice is for the kid (really his parent) to pay for being unable to attend.
You would have to be very petty to do so, considering the invoice and it's format.
The invoice was from the one parent to the other, the "ski hill" had to be paid in advance, so I doubt they got "stiffed."
Really it's stupid, the entire situation, including but not limited to a prepaid non-refundable cost per headcount for a birthday party for small children.
But I'm not sure what's worse, that the parents bothered to send an invoice for such a low amount, basically being butthurt over a no-show, the way it was done, or that it was published in anything.