Sensing items on a printed page - suggested technologies to look at for on/off, true/false, etc. detection
Oh. Simple. Ultraviolet reactive ink. Stylus has a large hollow tip that has a small uv diode with visual sensor of some sort that won't work on just uv light reflecting. Since UV reactive ink glows in the visible range when uv is shined on it, a digital color sensor would work.

Hell, skip the electronics, and just use a uv flashlight or stylus with just a smd uv diode at the tip, turns on when pressed down, the kid sees the picture glow.