J.W.
Sounds just like the virus my son had last month. The BRAT diet didn't work to clear up his diarrhea, so we bought him the kid's Florastor from Walgreens (behind the counter at the pharmacy), and it cleared it up in 3 days. That could be an option for you if the diarrhea still isn't clearing up. I don't think it's from the sand, kids just pick up viruses everywhere, and the viruses do have a gestation period. Good luck!