You need grids or something inside the cage to make the sides higher. The edges are usually curved on these pools so there's no real way to put something on the outside without leaving a gap that they can fall into if they try and get out. I've used to use kiddie pools as a temporary holding pen while I cleaned cages. They jumped out with little effort. One of mine always jumped out as soon as I put him in. The sides need to be about 14" high to keep them inside.
Ok I will add that to the outside thanks so much I forgot that they might jump out.