I don't know much about this, but when I was a child I also had a rash all over my body after swimming. It went away after a couple of days and it didn't came back. I didn't had anything for it. Maybe your child have the same, but maybe you are right and it is something else. I have to say we don't have a lot of worms and stuff in my country like in some other countries.
Good you want to find this out.