Try the Murano Island, but on an organised trip.

There are glass molding and sculpting demonstrations.

Also there is a nice park in the south of Venice: Parco delle Rimembranze.

If you visit the Doge's Palace, Cathedral or other famous attractions and you have kids be sure to search for Skip the line options. There's no fun in staying in a line for an hour with a kid with you.

Also Burano is a beautiful island where all the houses are painted in vivid and random colors. I would raise my kid there if I was italian :).