Hi! My Children Have Swam In Our Pool For 3

Hi! My children have swam in our pool for 3 years now. This year for the 1st time they are breaking out in a bad rash in between thier legs and also where thier life jackets rub thier arms. I put desitin on the rash and it burns them for a minute and it will normally be healed by the next day. But it keeps happening this summer. Any ideas what it could be?