This is a little bit away from the topic at hand - but a useful experiment I think kids should see...

Once a kid is at the age where he general has an idea of what metals are, and has the slightest knowledge of what iron metal is

Take some cereal, like bran flakes or something else with iron in it, put a serving or two in some water, blend it pretty well, then take a magnet to the blender cup to show him the non-negligible amount of iron that can be seen attracted to the magnet.

Then kinda be like... yeah - that metal you know about? Totally part of what makes your body work day to day, kid. Literally in your food and such.