Now that I'm thinking about it, I'm reconsidering one point: Seeing St. Peter's Basilica. Taxi there and back shouldn't take too long, and they could see the Pieta. (That will spare you from hearing them chide you years later--How could you take us to Rome and not . . . even . . . take us to see the Pieta?) To say nothing of the astounding vastness of the Basilica.

No one so far has mentioned the Pantheon. If you're in the neighborhood there. . . ?

But yes, definitely have them check a guidebook and tell you what they prefer to see. And if you yourself would like to see the Sistine Chapel, round them up, tell them they and you are going. . . .

(DH reminds me that we did both Vatican Museum/Sistine Chapel and Colosseum in one day, on our first visit to Rome. I say yes, but these are teenagers! I can't see teenagers wanting to see much Raphael or even Michaelangelo. After college, after they have had art classes, yes. . . . But opinions differ, people differ. YMMV)