Reservations in Switzerland are not necessary, a waste of money IMO, except for the few panoramic express trains.

Reservations in Italy are mandatory for the fast trains: Frecce and Intercity. They can be made online via ticket.oebb.at. Do NOT use "Seat reservation only” but add the Interrail/Eurail discount and select "One-way tickets and day tickets”.

Children under 5 travel for free but will need to sit on your lap if all seats are taken. If you want to book seats for them, then that is possible for the same price as for an adult. You'll also need to add a free child pass when ordering.