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.
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.