Try the amazing Technorama in Winterthur. The kids will love it. At 11.30 and 2.30 they do live demos of gases and electricity. The gas one is great fun, but the electric one is great because if you stay behind after the demo they let you "touch" a lightning bolt and freak out your hair on a Van derGraaf machine! Ask at both demos for an English language handout. Easily enough for a two days visit.
The galleries in Winthertur are stunning. To make it interesting for kids, do some research and get them to do a treasure hunt finding pictures containing certain objects or get them to copy a painting they like.
If the weather is good, go up the mountains at Pizol
- Wildhaus has a great toboggan run.
On a nice day, do the Heidiland walk in Maienfeld. We have done it with a 6yo, even starting the walk in Bad Ragaz. Talking of Bad Ragaz, which is also in canton St Gallen, the spa is amazing and pretty kid friendly.