A friend of mine brought up the idea of organizing a "Parent Camp," and it was such a great idea that I just had to share. Summer camps seem to cost a fortune, and personally, I've had to piece together several different camps so that I can be at work. Even if you're a stay-at-home parent, this could be a welcome a break/change-of-pace for your kids. Here's a great, low-cost option - Parent Camp.
This will take a bit of planning, but one parent would watch ALL the children for a day. It sounds daunting, but I find that kids sometimes do better in groups. Hours can be negotiated but our group was planning on doing 9-3. The parent in charge is in charge of keeping the kids "entertained" and safe, feeding them snacks, providing a lunch which can be pizza delivery, sandwiches, pasta (whatever the parent in charge can manage).
(it's likely that the kids will want to just play with one another, but if you want to plan some things, here are some ideas)