Hello jatin.shah01, and welcome to the forum,
All in all Florence being land based makes the airport > city transfer easier versus Venice. If you could fly into Bologna that's closest but Bologna is only 35 minutes from Florence by fast train.
From Florence airport it would be easiest to take two cabs - I suggest one or two adults in either cab and the kids mixed in between - or you could take the tram but cost and convenience-wise I think cabs make more sense. Staying close to the train station makes sense if you're heading out early the next day. Florence is small so even near the train station is still "in town" unlike Venice.
Venice is a little further but if you are just using Venice as stop over and don't care or have time to explore the city you could also choose to stay on the mainland part of Italy - maybe even pushing onto Padua/Padova which would get you that much closer to Ravenna. You don't say where you're flying from so I don't know how tired/jet lagged you will be but if both places are just quick overnights then you could bypass the logistical issues of Venice proper.
From Venice airport you'll need to take the bus or cabs towards Venice proper. Again staying near the train station it is an easier transfer the next day. But if you are going get on a bus anyway there is a bus from the airport to Padua which is on the fast train line towards Ferrara/Bologna and you could overnight there that much closer to Ravenna.
I love Venice and Florence is world class destination but if they are just overnight stops to you on the way in and you won't be exploring either city staying someplace practical overnight seems reasonable.
You can buy fast train tickets ahead of time you can save some money by committing to a specific train.
One note - unlike most Italian train stations in Bologna the fast trains come on level -4 and you need to transfer to the Level -1 or the surface to change to the local trains so allow time and pay attention to where you need to go.
Have a great trip,
=Tod