All a ten year old has to know is that white light is made up off all the colors together, and that raindrops act like prisms to split it up. Demonstrate with a garden hose in the sun. Few of us have actual prisms to show, but that's bonus points if you can. Point out that the sun is always low and behind you when you're looking at a rainbow, and that's how you know where to look for them.
The best demonstration I saw as a kid had a prism splitting a beam of light into colors, and a differently-placed prism recombining them into a beam of white light.
Be careful not to overthink this stuff. Watch for the eye-glazing that tells you your kid is getting Too Much Information. For best results, pick up a few general science books with lots of illustrations and leave them laying around.
Space/Science » in reply to Dumb it down boys.
Demonstration
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