G.W.
Wow, that's a really good question and one I probably never would have thought about. I think it's fine to ask the parents to pay for themselves or chip in. Just explain, "I can't foot the bill for all of it" or "I will pay for all the sodas if you can just chip in for the pizzas". I really think, if people aren't willing to chip in, then just skip the party or have a potluck picnic or something. But don't take offense if no one offers - they may not have given it a second thought but would be really willing and ready to give some money if asked.