>Crystal set its where I started at about the same age... a coil
>a capacitor a diode and an earpiece. (I still have a cats whisker
>in my junk box (primitive diode)
>Never too young

I would suggest a crystal set too. Wind a coil on a large tube and find an 
old tuning capacitor, a germanium diode and a pair of headphones. Kid will 
have many hours of fun using something they have built. That was essentially 
how I got into 'electronics' in mid primary school years, after getting 
books on 'radio' out of the library which had the instructions on building 
the crystal set.

Along side this would be doing the kit type things others have suggested, 
and if they do the right things then an op-amp can become a headphone 
amplifier for the crystal set. 

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