What it teaches
The planets in order from the Sun, and the facts that make each one memorable — the biggest, the ringed one, the red one. Also what the Sun actually is, which surprises most children: a star, and not a special one.
What to look for
This is the subject children most often already know something about, and the one where a wrong answer is most likely to be a half-remembered fact rather than a guess. Worth asking where they heard it.