This small park with a playground is near the Lindenhof in Zurich, with a slide, climbing ropes, grass in the sun, benches and a nice view of the city.
Since it's enclosed with walls, it's easier to keep track of your kids while they play.
There is a public toilet at the entrance to the park.
Location: near Rudolf-Brun-Brücke tram stop. See location on Google maps.