Is anybody keen to have a read of my draft and maybe edit bits of it into 9 Y.O. speak if I've got carried away?
Mike - I'd simplify it further.
Though good to relate it to something kids would be familiar with, the swing analogy didn't do it for me. I see the swing as an oscillator whereas a tuned circuit is more a gate that only passes certain signals.
It could be worth simplifying some of the component descriptions. While the one-way diode description is correct the main thing that's important here is that it demodulates the RF signal, leaving only AF, but I haven't found an easy way to describe this. Plus I've skipped over frequency though have described tuning.
Try this (omitting the Ancient Greeks and mangling Franklin, Faraday, Hertz & Marconi):
People have always been curious about lightning and thunder. A smart American guy flew a kite during a thunderstorm and got an electric shock. He was lucky not to be killed. But it showed that lightning was just electricity jumping between two charged parts of the sky.
Years later some other smart people found they could generate electricity themselves. That let them do experiments without having to wait for the next storm. Some made a special type of generator called a transmitter. If connected to a piece of wire, called an antenna, it could send invisible signals across the air. The signals could be picked up with another antenna, connected to a device called a receiver. Later on people worked out how to send sounds and pictures via these signals and broadcasting was born.
Radio broadcasting stations are transmitters, sending out waves of signals through their antenna.
This kit has the parts for the simplest radio receiver you can make.
There are no batteries. All the power that drives its earphone comes from the broadcast station.
You don't have to pay them but must listen hard for a signal as it is not very loud.
This type of radio is called a crystal set (after one of the parts, called a crystal diode), and works like this:
Signals arrive from the long wire antenna you have outside.
There are thousands of signals jumbled together, including radio, TV, mobile phone, wifi, etc, but you want to hear only one.
The antenna wire is connected to special parts called a coil and capacitor which together select only the signal you want.
If you make either the coil or capacitor bigger or smaller you can select another station. That's what the tuning or channel change button is for on your radio or TV.
The selected signal is at radio frequencies, which is so many vibrations per second our ears can't hear it.
This is why we need a crystal diode, which strips off the radio frequency and leaves only a sound frequency.
The sound frequency has fewer vibrations per second but it's still an electrical signal so we can't hear it.
However sending it to the earphones allows you to hear the voices and music as transmitted by the radio station.