Fingers in the nose
The teacher must get the child to always keep his fingers down when they are on the neck.

The teacher must get the child to always keep his fingers down when they are on the neck.
Put your finger in your nose without moving the 4th finger.
When the spike slips a lot, it means that the arm is pulling backward and the fingers are not flexing properly. Try resting the spike on a carpet and see what happens, above all if the child holds the cello with his knees. If the spike keeps slipping, the cello should be held away from the chest.
The question is very simple and the child is sure to react (a clock can be drawn on the child’s wrist with a biro).