A parachute game
The children sit on the floor with their feet underneath the parachute. One child is selected to be the shark and goes under the parachute. The other children then shake the parachute. The shark selects it's victim and grabs them by their ankles. That child must then pretend to have been caught by a shark and go underneath the parachute in as dramatic a way as possible. The original shark takes that child's place. And so the game continues.
Alternatively, you could have two lifesavers who can save a shark's victim by getting to them before they disappear under the parachute.