What it teaches
Reading an analogue clock. The hands are the whole difficulty: the short one moves slowly and means the hour, the long one moves quickly and means the minutes, and they look nothing alike in importance despite the long one being easier to see.
Starting at o’clock and half past is deliberate. Those two are learned as pictures, and everything else is built on them.
What to look for
The classic mistake is reading half past three as half past four, because the hour hand has already moved past the three. It is the right instinct applied to the wrong hand, and it is worth pointing at rather than correcting.
When o’clock and half past are reliable, Setting the clock is the harder companion game.