I would try bentonite clay...
http://www.bulkherbstore.com/Bentonite-Clay-Powder?id=tJCh9XYaAlso, perspiration. Get the child to sweat. I would guess your older dd is more physically active.
My sister and I were like this, she had skin issues, but I didn't. She stayed in the house and read a lot. I rode horses, built fence and worked the garden... even when I read books, I did it in top of the tallest tree I could find.