Two possible density experiments for kids are water density and floating an orange. These are simple enough for young kids and provide a visual representation of density.
The water experiment requires water, dye and salt. When water has different amounts of salt dissolved in it, it has different densities, so by using different food colorings for different amounts of salt it is easy for students to see which solution is more dense when the different solutions are layered in a clear container.
Oranges with the peel on are less dense than water, so they float. However, when the peel is removed oranges sink because they become more dense. This one gives a much more tangible example of density.