Backyard hose with a slow trickle will occupy for hours as they go around watering everything. The other kids hold small pails/watering cans to fill up and all take turns with hose. If u have any big icicles outside put them in freezer. In summer, put a little water in a kiddie pool and bring out saved icicles or some snowballs.
On a slightly different note, in the summer if u have any milkweed pods from nature walks, save them for winter. Right now my dcks are having a blast blowing the silky seeds around the room or watching them rise up from the heater. Very easy to vacuum up too.