Just discovered cats are using my yard as their litter box!?

We moved in our home a little over a month ago and have noticed cats around, but didn't think too much of it. We do have a cat, but he is indoor cat only so I know it is not him doing this. Usually I am outside with my boys everyday, but last week it rained a lot and we didn't go out very much if at all...it was either raining or just too wet to play. So today the sun was shinning bright and took the kids outside to play. We have a lot of bare spots in our yard and trying to grow grass which is another reason I probably didn't notice this earlier, then I saw it... we have like a dip in the ground from where the people that lived here before us had a fire pit and there was a bunch of cat turds in it. I have seen the cats back there before but I never thought they were "going" in there UGH. First off how do I clean up the area so it is safe for my children and second how do I keep the cats from coming back and doing it again??