Pose a situation with no clean answer: a friend asks you to keep a secret that worries you; you find money in the playground; someone is left out of a game.
Ask what they would do, then ask why. Then ask what the other people in the story would think about that choice.
Do not supply the correct answer, even when you have one. Add a complication instead and see whether the reasoning holds.
Moral reasoning develops through arguing about hard cases, not through being told rules. Asking how others in the situation would see the choice adds perspective-taking, and adding a complication tests whether the reasoning was principled or just convenient.