I would be a little worried about them picking up the gravel curiosity wise and potentially getting it stuck. If you use something like pea gravel, I would put sand over it rather than just the gravel, but you still run the risk of them eating the sand maybe? I would just go with all dirt and just sift it around if water spills since really, its not like when you have the horses in there and the fire hydrant pee is going to be going on