What should I use for my dogs potty area?

I have 2 small dogs (both under 7 lbs) that have a section on the back yard they are alowed to use for their bathroom. Currently it is gr covered but in the spring after the winter thaw, it is just a mess of mud, cleaning it up is nasty and muddy. I dont have an issue with odor at all, its just the mess I dont like. I have heard using rock is good but I am concerned about the rock considering my dogs paws are the same size as the rocks lol. I dont want them to injure themselves. I had also heard astro turf is a good surface but am not sure if urine would seep through when sprayed down (which is what I would want it to do) Anyone have suggestons??