How to stop children undoing their seat belts
Most children will eventually find they can undo their seat belt. So why don't they make seat belts that can't be undone by a child? This is because it must be possible to release the belt in an emergency, quickly and easily.
At worst this might be with a car inverted in a ditch at night, with access at arm's length through a window, or by a person in shock, unfamiliar with such equipment.
Any form of 'Chinese puzzle' is out. The catch cannot be hidden, must be obvious and accessible and must not require a large force to release it.
A different type of catch, perhaps a friend's seat, may not be undone as easily as one with which a child is familiar but this is usually only temporary. Given the right conditions they will soon get the knack.
Don't be tempted to make additional security attachments to the buckle. Anything like this will inevitably compromise safety and again tends to be only temporarily effective, presenting a new challenge rather than solving the problem.
Diverting the child's attention
Rather than trying to persuade your child that the buckle cannot be undone, try persuading them that there is no advantage in doing so. These are techniques you might like to try.
Show that the car does not go (or soon stops) with the belt undone.
The best time to start this approach is when you get a new seat (or perhaps when a car is changed). But remember to do it when there is no deadline for the journey.
If the child is old enough to really want to get somewhere, that's ideal. Try telling the child that unless the seat belt is fastened, they won't get to the party, zoo, etc.
A raised seat can also be some help, as it will increase your child's field of view.
A 'play tray' attachment can provide some distraction and may make the buckle less tempting.