Hi--
Are you sure it's exema>? Have you gotten any dr to look at it? Because I got stuff like this but only between my toes and only when I'd wear dark colored socks or shoes between the toes---I've heard that dark dies can irritate sensitive skin and ppl w/ sensitive feet are told often to wear white socks and then OVER that black, if they must wear black. You say it's where the sandal strap goes on her foot, right?
I had this problem, even in the summer when I wasn't wearing socks a lot, although I get this more in colder months, But the only thing that helped me was atheletes foot!