Eww. Sorry about this. But it's something you should know.
Health officials are warning about a rise in infections from a fecal parasite that can thrive in swimming pools. And yes, even when the pool's treated with cholorine.
The CDC says cryptosporidium, or more simply "crypto," can cause severe diarrhea for weeks. It says crypto infections spike during the summer months because people swallow contaminated swimming pool water. Unfortunately, the parasite has a high tolerance for chlorine and can live in a chlorinated pool for up to a week.
No easy answers on this one... sorry! Don't drink the water should be a given.. but with kids, who knows?
At least you'll have a pretty good idea what the problem is and what you should do about it.