I hate cats!!!!

Preston was having a great day today. He played the piano a bit...does it just crack you up that he is using his toy piano stool? Ha ha. We did some sidewalk chalk in the backyard. Then we went to the park and all is well until....dun dun dun...Preston fell on the steps going up to the slide and I ran to pick him up and when I did...dun dun dun...he had poop on his hands and was covered in short white hair. Pretty sure it was a freakin cat that left his little droppings. The hair was on the slide. I was so grossed out that I wiped him EVERYWHERE with an antibacterial wet wipe that I keep with me at all times. Pretty glad I had them. Hannah enjoyed everything from the stroller. She was just squealing and having a good ol time.
While we're on the subject...we have a neighborhood cat who thinks the rocks in our backyard is it's litter box. I'm not kidding you, we have so much poop in our backyard it's not even funny. I don't know what to do. I know whose cat it is. Some say I should call animal control, some say I should talk to her. But, in reality, if it's an outdoor cat how can they stop it?? What would you do? Preston wants to play back there all the time and I honestly can't stand the fact that it's there and my son wants to play. It could make him so sick if he got into it. I've just about had it. I've chased it out several times but there is so much back there...ugh. I'm open to advice. Help!