Locks, Latches, and Bolts

Safety

Check hardware weekly: any screw, hook, or knob that comes loose is a small part that fits the test cylinder (1.25 in. wide × 2.25 in. deep).
The key and its tag together must not fit the cylinder (at least 6 cm or 2.4 in. long), and the ring joining them must not open by hand.
Do not use latches or catches with small magnets, which are dangerous if swallowed.
Check hinges for gaps that pinch fingers, and keep child-proof locks on real doors, cabinets, and gates; practice on the board does not replace them.

1
Carry the box or board to the table and sit beside the child, with the door facing them.
2
Take hold of the knob of the bolt with your thumb and index finger. Slide it slowly to the side until it clears the catch.
3
Open the door slowly and look at the picture behind it. Seeing what is behind the door is the point of interest the first time.
4
Close the door. Slide the bolt back into the catch until it stops.
5
Invite the child to open and close the door.
6
On later days, show each new fastening on its own with the same slow movements: lift the hook out of the eye; lift the latch bar; turn the knob.
7
When the child stops, fasten the door and return the material to the shelf.

The door does not open until the fastening is fully undone, and it swings open again if it is not fastened.

First: a wooden box or board with one small hinged door fastened by a sliding barrel bolt, with a picture or small mirror behind the door
Later, one at a time: doors fastened by a hook and eye, a lift latch, a turning knob
Later still: a padlock fixed to the board by a hasp, with its key fixed to a wooden tag by a closed steel ring
All hardware screwed on firmly