Hello @aziz.baig24 

To avoid reservations, you can travel with regional trains from Venezia to Milano and change in Verona (time of travel 4 h 25 min) or you have direct trains with mandatory reservations.

From Milano to Interlaken, take a RE80 to Lugano and after an IC to Luzern (time of travel 5 h 12 min).

I wouldn’t risk a 5 min connection in Verona especially with a family. 

The next train is an hour later, so as long as it isn't too late in the evening, there's no problem and you can just go have a coffee. I've also noticed that the regional trains from Venice to Verona have a lot of slack in their timetables and can make up some time before they arrive in Verona.

I searched this connection in the timetable of the SBB (sbb.ch), the 5 min connection is not displayed, only the next train one hour later with a stopover of 65 minutes. The travel time of 4h 25 min is with this this long stopover.

Trenitalia.com shows the 5-minute connection.

In Switzerland, you have a minimum connection time defined for every station.

In Italy, you have probably also this minimum connection time but I suppose that Trenitalia don’t communicate it to other railways and the SBB chose themselves this time longer as Trenitalia, perhaps to avoid complaints of customers who miss the connections.

Or Swiss people are slower than Italians…

Every station that offers connections has a minimum connection time defined. It's included in the timetable data. If sbb.ch shows something else, then that means that SBB deliberately chose another value (which does surprise me).