I would definitely charge either a deposit or a holding fee if you think you can get it. None of my families would go for it though. I knew that I'd be facing this problem this year, so I've been putting aside a portion of my pay every week into savings to be used during the summer. I figured out how much I'd need weekly for groceries, gas, some "fun money", bills, etc. and made a savings goal. We've got three weeks of school left and I'm only about $100 short of my goal.
It's going to hurt spending the money that I've worked so hard at saving all year, but it's better than having none.
I know that's not helpful for this year, but maybe for next year?