This is a great challenge, but it would be a better challenge if you asked people to draw them without their hair. That is a key distinguisher, and in a lot of modern cartoons (predominantly anime) it is the only way to tell characters apart. Without hair, it would force people to really look at the facial structures of their characters, without relying on hair to make them original or different looking.
Not necessarily, anime artists aren't as limited as many people believe. The noses, eyes, face shape, and of course body shape all have plenty of room for variety. Some just don't recognize the options they have.
Seems like a decent way to get kids to start drawing characters consistently different.
I remember back in the day, a young lady came up to me, saying that the girl in her anime picture was really another girl wearing her clothes. I said to that young lady, "Why, how could you ever tell? Given their similar facial structures and body shapes, one could easily pose as the other. It's not intentional, but merely your skill with drawing that facilitates such an error."