It gets really hot in Vienna in the summer and this is a lovely place to come and swim and hang out like the locals do. Shaded by trees with a wonderful lawn, it's a great place to go on a hot day with kids.
Green place at the Danube close to the city centre, where you can go sailing on small boats, or rent a paddleboat, or go swimming. The Ufertaverne is a nice restaurant infront of the harbor
If its to hot in the City, a trip to the alte Donau is worthwhile, especially when renting pedalboats and electroboats and then diving into the water
Swim in the huge water paradise or rent an electric or paddle boat and enjoy the peaceful surroundings (or put on some ice sleds in winter when the area is frozen)