Brilliant!Many thanks.

I am kane. Thanks for the tip about slowtrav.com. What a great site.

I'm sorry you have taken Ciceron's last advice negatively, because it is actually a very helpful one coming naturaly from someone who knows Venice. Most posters replied with ideas about cities out of Venice "area" (Venice proper + other islands, Murano, Burano, Torcello, Lido, etc). I think they thought you already considered those islands. Fortunately Ira suggested them and Cicerone's post followed. You can be a bit more open minded and try following Cicerone's advice. I can easily imagine your kids would enjoy those island hopping on waterbus.

Forgot to say, these island hopping matches your request "short" day trips more than those to Verona, Padova, etc.

Prhirsch:

Murano is close; about 20 minutes. Your children might enjoy visiting the glass foundries, but be prepared for a pretty hard sell.

As for the other islands:

You can get to Padua (about 30 minutes) faster than you can to Burano and Torcello. I'm not sure Padua would be very interesting for children, though.

You can also get to Vicenza in about the time it takes to get to Burano and Torcello. Again, I don't think it's a place that would appeal much to children.

Ditto Verona: About an hour with a fast train, not much longer than to Burano and Torcello. The castle (now a museum, but still interesting as a castle) and the arena are more likely to interest children.

Thanks to everybody, these are great ideas. I think we'll see how everybody feels at the time and see what the weather is like and go from there.

Don't forget. Padua (=Padova) may be 30 minutes train ride (only station to station, may take a bit more to get to the places of interest) but waterbus ride to the islands itself can be a big fun to kids.