the swimming is better for them more than the chlorine is harmful. push it back out the skin with iodine. theres not that much running and playing and swimming later in life, especially if they dont do it now.
do make sure that they are fully hydrated so they soak up less of the water. drinks should be available while swimming. i like orange juice on hot days. others prefer fresh made lemonade. an herb tea with honey is good, too.