I would drive to a town on the rail route and get the train in. Venice has a multi storey car park but it’s not in the city. Arriving by train is better in my view. The water bus stops at the station.
I wouldn’t get a guide either. It’s so easy to get around and make your own decisions about what’s best for you as a family. Churches, canals, walking, museums etc are the top draws. Just decide which ones. Many guide books suggest walking tour routes and these work well. Also gondolas are expensive but you can cross the Grand Canal for a fraction of the price.