How do you stop a child from undoing their car seat buckle?

How do you stop a child from undoing their car seat buckle?

Take some toys or games they are able to play with in the car, but try to avoid heavy ones. If the child undoes the harness during the journey, pull over as soon as it is safe to do so and repeat that the harness must be done up to save them from being hurt. Do not start again until they are back in their harness.