I am traveling with my family of 5 in August, including an 8 year old, and I wanted to take advantage of the Familia offer. We plan on taking 3 specific rides, Rome-Venice, Venice-Florence & Florence-Rome. I want to take the fast Le Frecce trains. Can I do this purchasing the Familia tickets and can I do it ahead of time on-line or do I have to wait until I get there? I know the dates and times I want to travel.
I don't see Le Frecce trains specified at http://www.trenitalia.com/tcom-en/Offers/Familia-offer. I suggest that you just start a booking and choose the best price on the departure you want, if you're ready to commit to a date and time.
As you choose your trains, the "Offerte" tab should show where special deals are available. Or start with the "Offerte" or "Specials" (or whatever it's called in English) button.
You would have been much better off financially buying the super economy tickets 120 days out for yourself and your kids.
They may still be able to get Super Econ tickets, depending on the travel date. If available they will trump the Family offer, and are available on Frecce trains.