What it teaches
Check: the first rule in chess that involves two pieces at once. Up to here every question has been about one piece and the squares it can reach. This one asks whether one piece reaches another, which is a genuinely different question.
It is also the rule that makes the game a game. Everything about chess that matters — defending, blocking, and checkmate itself — is built on it.
How it works
A black king and one white piece appear. The child answers yes or no. Half the positions are check and half are not, dealt at random, because a game where the answer is always yes teaches a child to say yes.
When the answer was yes and the child said no, the squares the piece is attacking light up, so the line into the king can be seen rather than described.
What to look for
The positions have only two pieces on purpose. Blocking, defending and getting out of check all come later; the question here is only whether the king is attacked at all.
A child who says yes every time will drop to about half marks quickly, which is the most useful thing this game does.