Buy a mint-tin cmoy amp and you'll see the same thing. The circuitry is very simple. Wires from the input connect to an op-amp chip, which is connected to the output. Some resistors feed the output back into the input, controlling the gain. Finally, a big beefy capacitor sits between the power supply and op-amp to provide some extra current for sudden loud parts of the music. That's it. Everything cool happens inside that little IC.
Agreed - if I wanted something that'd make kids interested in what's going on, hiding all the "magic" in an opamp chip isn't how I'd do it. Something super simple, where I could explain exactly what each component did, and more-or-less how they do it (with the obvious handwaving requirement for explaining transistors to just about anybody).
Could be built for approximately zero dollars by just about anyone who's the sort of person who's got transistors "lying around" at home. I can easily imagine explaining how that works to the satisfaction of my "test" 9 year old. Not so much with the opamp version...
Here's a bog-standard amp which looks almost exactly the same as the encased-in-epoxy variant http://electronics-diy.com/electronic_schematic.php?id=797
Here's the information for the op-amp they choose:
http://www.ti.com/product/opa2132
Note that they don't even give you a schematic for the op-amp because it's so complicated.