why don't you try to find a babysitter for that evening. Ask the Hotel where you go to if they can help you .
You will enjoy it more relaxed and also if small Leonard does not like it (remember it can last 3 hours) or if he is not in a good mood that day (can be also very hot) , it might be not a good experience for you and for him either......actually I do not know whether little kids can get in really. I would inform myself very well before going there, would be a pity not beeing relaxed or not getting in, having also spent a lot of money for this beautiful event.
If you cannot find a solution, I can also ask a friend who lives in Venice if she knows a babysitter.
Ciao and saluti
Andrea
we have met in Lucca , can you remember

http://venezia.kijiji.it/f-Servizi-Baby ... tIdZ300110