Ciao Jansport, welcome to our forum!
How much can they walk? Do they just need to take breaks or will they also have some type of support, whether it be a cane or a wheelchair?
Driving to a train station and then taking the train into SMN train station is one possibility. You could do that by driving to Poggibonsi and taking train from there.
The other possibility (and likely better one) is to drive to Florence to the Villa Costanza tram stop, there is a large parking lot at the start/end of the T1 tram line located off the A1 motorway. From Radda, you will need to drive toward Florence either on the Via Chiantigiana or the Firenze-Siena road and then get on the A1 and drive in direction north to Bologna.
Villa Costanza seems to be an exit off the A1 but it is just to get to this parking lot. You can park there all day for just 7 euros (or 0,50 cents per hour), then take the tram all the way to the SMN train station (the stop is either Alamanni or Valfonda, depending on which side of the station you stop at). The tram costs €1.70 per person each way.
Walking around Florence can be difficult because some streets are uneven and some cobblestone, but there are places to sit down and take breaks in the main squares. If at any time they need assistance, you can always catch a cab, they are allowed to transit through the center of the city.