Can I add another seatbelt to the back seat?

Question

I have a bench seat with 3 seat belts, I have 1 more child than seats in the van. Can I safely add a fourth seat belt to the bench? This is a toyota sienna. From research I've been finding people are saying one seat belt per "seat", but why is that? I know double buckling 2 kids isn't ok, so I thought adding another seat belt would be a good solution.

Answer 1

Before doing this I would first check with your local police department to make sure that it will be legal and then check with your local fire department for assistance for the correct way to mount it.

Answer 2

check your compliance plate, it will say, 5/6/7 seater, this is the maximum amount of passengers you can carry.

To fit another seat belt would make the vehicle defectable without a vehicle inspection.

Seat belts are anchored at reinforced places,  

Answer 3

Problem would be mounting points for the seat belts. They don't like you to buckle to kids in the same belt for points of failure. So unless you tap new mounting points for the new seat belts it would do no better then 2 kids with one belt.