Materials to line kiddie pool inside?

I am going to get a large, hard plastic kiddie pool and fill it with gravel to use for planting. Im thinking that the gravel would put many punctures in the kiddie pool so I first want to line the walls with something to protect it. I was thinking maybe rubber sheets, but I'm not sure if that's a good idea. I'm looking for a type of material that is flexible, cost effective, and that will not fall apart in water. If anyone has any suggestions that would be great.