Last year I bought a jumbo box and paid for it myself. That way I had the ones I wanted, I had control over who wore them and when, etc. I still have some left over and I had up to 4 kids in diapers last summer!
If they are actually going to be standing or sitting in water (like a kiddie pool or what have you) I wouldn't have them go in the diaper. Too many times I have seen the diaper explode (the little crystals inside pop the exterior of the diaper) and that's a disgusting mess!! If it's sprinkler fun, and just pouring water from bowls and buckets etc then diapers should be okay.