Bill for NOT Attending a Birthday Party! What do you think?

You're invited to a birthday party and accept the invitation to attend. Sounds like a fun day of going to a ski area and doing lots and lots of tubing and having something to eat. When the day of the party arrives, your parents decided you won't be attending as they have other plans for the day. A few days later, you have a note to bring home from school from the host of the party. Inside, is a bill for about $25, the cost of the ski party. The parents of the birthday child say that all guests were told if they accepted and didn't show up, the fee would still need to be paid, so the parents felt that "no-shows" should have to pay the expense. The five year old boy's parents were very upset about receiving the bill in such a way and said they didn't plan to pay it.

YOUR THOUGHTS!
What do you think?Should the parents of the boy who didn't show up, have to pay the bill?
Should the parents have called the boy's parents and not send a note home in the boy's backpack?
How would you settle this dispute?

Gail