hey martica
stay on the island of venice, look at cross-pollinate.com, they have a 2 bed/2bath casa valentina in dorsoduro and 2bed/1bath near saint marks for $650 and below. read the fine print since some want cash euros on arrival. ask bank to up your withdrawal total. you can also check booking.com and put in filters to see what's available. take the vaporetto over to the lido, there is a beach there, don't know warmth of water but the kids can run along the beach. take them to some campos/plazas and they can play around, you'll get lost coming to dead ends, that's zigging instead of zagging. let the kids eat pizza and gelato and enjoy the views, night time when daytrippers leave, ride down the grand canal. make the most of enjoying what you have. buy things for breakfast and can eat in the morning before your adventure. have fun
aloha
ps some may have a washer, i bring tide pod or liquid in ziploc or bottle, plus a clothes line with pins, wash morning, hang to dry while out and about. sounds crazy but it works. that means you pack llight.