How could you demonstrate to a child that solid water is less dense than liquid water?

You could explain the densities of different forms of water by preforming an experiment. You can place an ice cube into a glass of water and watch it float. This proving that the frozen form of water is less dense than the liquid form and the reason for the ice floating.

For this, you need to first explain that less dense objects float in a liquid that's less dense than those objects - if an object is more dense, it will sink because the gravity is stronger on it.