The obvious answer would be Keratex hoof hardener, but you do have to be a bit careful with this on unshod feet, for example, never apply it to the wall, which is meant to flex, or the frog for obvious reasons. You also shouldn't use it on soles that have just come out of shoes as they are meant to exfoliate first, but I'm assuming your youngster has never had shoes. Like the other poster said though, you want to improve the sole with conditioning over surfaces, Keratex is only a short term solution.