Venice.2adults + 9yearold suggestions for hotel near all, as only have 2 days to do all we can. tips helpfull?

We will be in Vence arriving by air in theevening of the last Sunday in June . We are due to go to Frenzie/Florence by train for a four night stay then on to Sorrento again by train.
All info help on accomodation and reducing the cost of train travel with somesort of family ticket or travel pass.
Our accomodation in Sorrento is happily booked as is our flight home from Naples.
In Venice the ideal would be a a large room with a Double and a single rather than the usual fold up or down second bed.
As time is a premium close enough to all the sights and the transport links yet not noisy or unsafe in the evening. Naturally all this at a price that does not reflect the stories of how expensive Venice is.
The best times to see what in terms of crowds and the light will be welcomed.