She started with a cherry tomato, which quickly sank to the bottom. It looks like an oval shaped egg yolk in this picture due to refraction (bending) of light through the water/glass/air transitions, but trust me, it's a tomato. Then she tried a piece of bread, which floated, but then changed it to a wadded up piece of bread. It sunk! By smashing the bread into a tighter space, the density increases and becomes greater than that of water. We tried several other food items too, including cheese!
So try it out. Let your kids choose what to drop in the water, but have them make a prediction of the outcome first.