What it teaches
Capturing, and the part of it children miss: a piece can only take what it can already reach. Knowing how a bishop moves and seeing that this bishop cannot get to that pawn are two different steps, and the second is where real chess begins.
It also teaches blocking without ever naming it. A rook two squares from a pawn cannot take it if something stands between them, and a child works that out here rather than being told.
What to look for
Ask why the other two are safe. That question is worth more than the answer to the one being asked, because it is the question a player asks every move of a real game.
Play How the pieces move first. A child who is not sure how a bishop moves cannot possibly work out what it can take.