C.S.
I would bring outside toys for all the kids. Walmart has a great selection of water toys, slip-n-slides, water balloons, bubbles, frisbies, kites, super-soakers, etc. You could get a large beach tote for each family group and fill it with all sorts of toys for the backyard/park and include a slip-in-side for each family (something the older kids would like as well). You could include some dollar store items to fill it out without looking "cheap". Good luck and have fun!!!