Hello Meet_kang
If you are purchasing tickets from Zurich to Milano Centrale and there is no mention of the Saver Day Pass under the Discount cards, it means that this option is not available for this specific route. The Saver Day Pass is only valid for trips within Switzerland.
Regarding your 4-year-old kid, according to the information provided for the different train services, children under 4 years old generally do not require a ticket and can travel for free. Therefore, you do not need to add your 4-year-old kid to the passenger list for the same route.
I wish you a nice evening.
Greetings
RahelW