Here I must disagree with Lourdes for three reasons.
First of all, whenever there is an important canonization Rome hotels generally up their prices.
Second, if you are staying in Florence for a few days why accept the hassle to find another hotel for just one night?
Third, their are many Frecciarossa, Frecciargento and Italo trains which depart from Florence SMN very early still giving your parents plenty of time to get choice locations in St. Peter's Square.
For example, leaving Firenze SMN at 07.38 you arrive at Roma Termini at 09.10 which I believe would be early enough.