As an Italian, I would have no problem walking this - nor would I worry about my family walking it. Its a straight shot from the train station and then one left turn. Sunset isn't until at least 8 pm so that makes it even easier. I'd guestimate maybe 10 minutes?
However, the area around the train station/S. Maria Novella is favored by pick pockets/beggars - precisely b/c of the # of tourists getting off the train and wandering around somewhat lost. If you feel like this is going to bother you/make you uncomfortable - then take the cab.