Start with a sharp knife, not a dull one. Teach the blood circle, cutting away from the body, never catching a dropped blade, and closing before moving. Supervise real tasks rather than lecturing, and let them own the knife so they take responsibility for keeping it sharp and clean.
The blood circle: hold your arm out and turn in a full circle, and nobody should be inside that space while you cut. Cut away from your body and away from your supporting hand. Never catch a falling knife. Close or sheathe before you move.
Four rules, learned properly, prevent almost everything.
Handing a beginner a dull knife feels cautious and is the opposite. A dull edge needs more force and slips unpredictably. See can a dull knife be more dangerous.
A sheepsfoot or drop point in a modest size is the right first blade, not a large or pointed one.
Give them a real job: making feather sticks, preparing food, opening boxes. Skill comes from repetition under supervision, not from being told.
Make sharpening and cleaning their responsibility. A person who maintains a tool respects it, and that is the durable part of the lesson. Rules on age vary by state; see minimum age.