How early should I (and my family of 5)arrive prior to my departure time from these stations? Thanks.
We generally tried to arrive at least 15 minutes before a departure in order to find the correct track.
The train station in Florence is not too huge, but if you're walking there from the city there is a fairly long tunnel that takes at least 5 minutes to get through.
Rome Termini is bigger, but not unmaneagable. If you are taking a night train I would allow a few more extra minutes as the trains are very long and sometimes it takes a few minutes to find the correct car.
I agree with Jill.
Also, the Eurostar will have a diagram posted, you can look for the location of your assigned carriage number ahead of time and have a general idea of where to be standing when the train arrives.