There is a train leaving Rome Termini at 12:36pm and every hour afterwards. It takes 4 hours to get to Venice. There is one at 11:35am you might be able to catch. Depends on you disembarking process time frame. 70-100 euros each.
You could fly as well, a little more money and a later flight. The train is much better. You can move about much more freely and relax more comfortably. Plus you have something to see out the window! Enjoy.
PS Limit your luggage to two small and a carry pack. Watch for pickpockets and bothersome people in Termini. Only trust people in an official uniform.