When even Rick says that rail passes rarely make sense for Italy, you know they are a bad deal. Even full priced tickets in Italy aren't that expensive, so it's very hard to make a rail pass pay off.
All of your routes will be served by high speed trains. On all but your last trip, you will have a choice of Italo http://www.italotreno.it/en or Trenitalia http://www.trenitalia.com/tcom-en. For Venice to Milan, Italo goes out of your way (via Bologna), so only look at Trenitalia on that route.
On both of these companies, the cheapest tickets bought in advance are non-refundable and non-exchangeable. If you don't take the EXACT train you are ticketed for, your ticket becomes scrap paper. The midpriced tickets can be "cashed in" for another ticket, but you pay any difference in price (in other words, you don't lose value). And the full priced tickets can be exchanged or refunded before travel, so there's no advantage to buying these ahead. Once the cheapest tickets for a train are gone, they don't come back. Full priced tickets are available until just before travel, unless the train sells out first (rare, but there are some reports of that on busy travel days).
Go to the websites of the train companies I gave above, and search for trains on your travel dates. Be sure to look for 4 tickets together and put in your childrens' ages. Use the Italian names of the main stations, even on the English language websites:
Milano Centrale
Roma Termini
Firenze Santa Maria Novella (Firenze SMN)
Bologna Centrale
Venezia Santa Lucia (Venezia SL)
If you don't mind the full price, you can wait until you get to Italy to buy. If you see a discount ticket and are sure of your travel schedule, buy now to get that price (discounts will definitely sell out).
EDIT: I see I was cross-posting with several others. You see that we all gave the same advice.