Viator is a 3rd party reseller of tours and is owned by Trip Advisor. I'd use them for research and if something looks interesting the name of the provider is given, usually under "more options", lol. Book directly with the company providing the tour.
Honestly, with just 2 days (3 nights, right?) in Venice, I'm not sure I'd spend a day going to Murano but if that is a must for your group then by all means do it. I would not book a gondola ride, I'd just turn up at one of the gondola points and get one when it's convenient for you.
For Rome - use Rick's Heart of Rome walk for the Pantheon, Trevi Fountain and several other interesting sights in the area. The Pantheon is free. You can easily do this one evening and stop along the way to eat dinner. I enjoy the Miscelanea for dinner which is right behind the Pantheon. I was first taken there by a Rick Steves guide in 2014 and have eaten there a couple of times including last October. In fact I'd separated from my Best of Italy group to go do stuff on my own and as I was finishing I looked up and saw my group outside with the guide pointing at the restaurant as a possibility for dinner. I laughed, waved and about half of them came on in. Great pictures on the wall of US presidents etc who have eaten there, plus they still have a pay phone installed on the wall. (Not sure if it works).
https://d3dqioy2sca31t.cloudfront.net/Projects/cms/production/000/023/487/original/8641aa9526168b2ac4cfc60dd140d7b5/heart-of-rome-walk-map.pdf
http://www.miscellaneapub.com/
I probably would not add anything to your Rome days as they look pretty full. Do the Colosseum/Forum/Palatine Hill one day. Do the Vatican Museum/Sistine Chapel another day. Both are pretty exhausting areas so I'd not tack on anything big to those days.
What a fun and memorable trip for Nana/Pops and the kiddos! You'll need a vacation when you get back!
editing to add: Everyone has quicker fingers than I do today! There were no replies when I started typing, hahaha!!