Are you looking at RailEurope? They are more expensive than buying form Trenitalia.
The Trenitalia price for a ticket from Venice to Rome on the ES train is 99 euro in 1st class and 73 euro in 2d class.
Then, with the "family" offer, you get a 50% discount for the child's ticket and 20% discount for the second adult (the other aduult pays full fare).
You can buy these tickets when you get there, a few days in advance of departure. Unfortunately you can't book on the Trenitalia website with an American credit card.
Here is the information about the family fare:
http://www.trenitalia.com/cms/v/index.jsp?vgnextoid=91fb335a9a605210VgnVCM1000003f16f90aRCRD