That's a pretty ambitious agenda, esp. if your parents have any sort of health/mobility issues.

You'll get a lot of suggestions about your itinerary, I would go for two nights in Venice, leave 1st thing in the am for the train to Florence and stay just one night in Florence, you'll still have plenty of time and it will give you primarily b/c having two different cities, two relatively lengthy train rides + two different hotels on your first two nights means that you'll have very little opportunity to take in the locales, rather you will just be experiencing a lot of public transportation.

As an alternative, forego Venice, stay one night in Milan and go straight to Florence.