Family of Five to be based in Zurich + Venice

Hello schellgirl! If you'll be making rather short day trips from Zurich, your best option is most probably a Swiss Half Fare Card for the adults, which grants a 50% discount on regular tickets. You can order along a free STS Family Card for your daughters, which allows them to travel along for free with you on all trips in Switzerland. The Swiss Half Fare Card will also be valid for the Swiss part of your trip to Venice. This means that you'll have to purchase 50% discounted tickets for the adults from Zurich to the Swiss border (your border station will be Chiasso if you pick the most direct train connection), and regular non-discounted tickets for all five travellers for the Italian part of the journey. You don't have to switch trains at the border, all you have to do is show your Swiss Half Fare Card and discounted tickets at the Swiss leg of the journey, and the Italian tickets at the Italian leg. Such an international train trip requires seat reservation.

Some tips on spending your time in Zurich can be found in this previous topic. The Zurich Tourism website will help you out as well. Many day trips are listed here, including childrens' activities. There's a specialized chocolate shop in Zurich, but it's also possible to visit the Schoggiland chocolate factory in Flawil, which is at a one hour train ride from Zurich.

The number of direct trains is limited indeed. But connections from Zurich to Venice with one change take just 6.5 hours and can be found in the timetable.