Hi, we are a family with 2 young kids living in the Rheintal and over the last 8 years we have often invited family and friends over with their young kids. Typically people with young kids preferred to come May-June or Sept as in the high summer months it can actually get a bit too hot especially for the under 2s and also - why have your holidays when the schools have theres unless the kids visiting are already in school and then you'll need to fit with their timetable anyway. Pleaces we liked to visit are in our area rather than Zurich where I think you are, however I'll tell you anyway :-) In Liechtenstein: Vaduz to see the castle and Malbun for mountain walking, also Erlebnispark is a good working farm and some kids like the go karting or pony riding. In Rorschach for a rainy day is Fägnäscht
http://www.faegnaescht.ch/home.htm indoor playarea, there is Connyland with fun rides and dolphins, in Gossau, nr St Gallen the Walter Zoo
http://www.walterzoo.ch/ can fill a whole day and you can bbq your own food or eat there,
http://www.knieskinderzoo.ch/ Swimming at the lakes is alwys fun for out of towners, I think there must be many possibilities in Zurich, we like Caumasee on the way to flims-laax, just over the border in Austria there is the Feldkirch Wildpark - plenty of shade and free entry, Inatura in Dornbirn, there are indoor swimming places like Alpamare - Pfaffikon
http://www.alpamare.ch/ and Santispark - St Gallen
http://www.saentispark.ch/, if you want to drive a bit you can visit Europapark (a huge hit with parents and kids!)
http://www.europapark.de/lang-de/c51/default.html , there is also another aquarium type park near bodensee/konstanz but I can't recall the name, we went once and it's also ok, as is insel minau near konstanz. Hope I could help! Enjoy the summer