Very good question! The Dreirosen Freizeithalle maybe a good place to find other babies and children to play with (in the soft toys part), or to enjoy some trampoline jumps ;-)! Take your family pass along, you will save in the trampoline (the soft toys are free).
There are always plenty of kids when it rains so use your lunch time if possible -the quietest time to go there - ah, and there is also a restaurant/cafe where you can find simple meals or snacks with reasonable prices

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By going there you will also be helping the project itself which helps teenagers, you will see them working!
http://www.dreirosen.ch/freizeithalle.html
Have fun!