The Artesia overnight sleeper service operates directly between Paris Bercy and Venice Santa Lucia, leaving at 1855 and arriving at 0939. You can book 6-berth accommodation for your party for a total of �137.50.
The Artesia overnight service to Rome Termini leaves Paris Bercy at 1859 and arrives in Rome at 0951. 6-berth accommodation will cost you a total of �254.
Those times and fares are based upon travel on 17 July, for 2 adults and 3 children under 12 (although it looks like you'll pay the same fares irrespective of the composition of your group). To check the details for your intended date of travel, go here:
http://www.sncf.co.uk/
If you're unfamiliar with Gare de Bercy, information about how to find it is here:
http://www.raileurope.com/us/train_stations/pa risgarebercy.htm
Chris