While I don't know of anything battery powered and safe along those lines possible it could be really scaled down to an audio modulated LED? Maybe something along the lines of a low voltage (12 volts) color organ scheme. A few transistors, a cheap microphone and a color organ where if he whistled and changed frequency of his whistling the lamps would light differently. Simple circuit with incandescent bulbs could be built. Somewhat interesting, battery powered and safe for the average 9-10 year old.
Just My Take
Ron