How about a portable shredder right in the store, with a nice big glass window on it? Bring your phones in and let your kid drop them in the shredder and watch the snap-crackle-pop, and get paid a nominal amount for each one!
Medium value stuff is shredded into a big pile and then they leach the gold off it with cyanide. It has a much higher concentration of gold then raw gold ore.
Low value stuff is shipped to poor countries where the copper is extracted by burning and picking through the ash. Very toxic process.