LOL, careful what family members can tell you. I know I'd take advice from some of mine with a grain of salt! But If you don't believe us, then read the many comments from previous guests regarding location, such as....
"Fantastic home and location. Super convenient, 3 min from train station and right in the heart of everything in Florence."
The apartment is in the city center.
We stayed much further away from S.M Novella than you will be (2 blocks north of Museo San Marco) and walked from the station and to everywhere else in Florence from that location. Your offspring are adults, same ages as some of the many backpackers on their summer adventures abroad, so give 'em a map and let 'em roam. On my own or with another young person or two, I bopped around parts of some European cities at barely 18 without incident years ago so they'll have fun.