Dogs not allowed

In front of my house there's a playground for childeren. With a couple of swings, a slide, a climbing frame and a climbing tower. As any smart man should know dogs should not get there en least of all do their "thing" at the lawn. Lots of kids run and play in that little "parc", so it's got to be save and clean... That's why the community placed signs like this. To let dogkeepers know they have to find another place to take out their dogs. When I got home from work I saw someone walking in that same parc, with a dog, and it was doing it's "thing". Since menkind is very agressive these days I didn't dare to say anything about it. But I took a picture of the sign....The guy with the dog? He saw me, so I hope he got the hint.....