Italy is awesome! It is hot in the summer, so something to keep in mind with elderly folks. We actually did TB instead of
ABD when we went b/c it worked better with our schedule. That said, I agree that it's hard to go to Italy and not hit the trifecta of Rome, Florence and Venice. If grandparents are into art and churches more than outdoor-sy stuff then I'd go with Rome/Florence/Venice (or if they're like my mom, who likes to hit the high points "Uffizi, check, Sistine Chapel, check, gondola ride, check...").
From my TB experience, it's the little things you notice when traveling with ABD (such as euro for the restroom, tissues for the restroom, dividing kids up and sending them with one Adventure Guide while the adults stayed with the other Adventure Guide, a movie on long bus rides, etc) that aren't there on TB. Hard to quantify if that's worth the price difference, for some it is, for some it isn't. The biggest surprise for me was when we went in the Etruscan caves in Orvieto; not on my radar but we had an excellent guide and it was an interesting experience.
With TB in Venice we were a quick walk to Piazza San Marco from our Westin; ABD uses a Hilton that has a water shuttle. In Florence, TB stayed at the Westin a quick walk to the Ponte Vecchio, ABD is in a beautiful Tuscan countryside hotel. My understanding is that ABD uses hotels which are truly family friendly (think swimming pools). Since I have a pool in my backyard, but not the Arno river or Venetian lagoon, it's less important to have that when I travel (but of course everyone is different!).
I think both ABD and TB are excellent family tour companies. Honestly for me I look at: dates, itinerary,lodging, tour company.
Have fun planning
