It's a cute experiment with the kids, and i'm glad they got away without injury -- but on the other hand i've witnessed first hand what a lead battery that is floating around the cabin can do during a wreck -- and what I witnessed was, sadly, deadly for the driver.
single orientation nylon straps that are strapping heavy square objects against foam cushioned seats and sandwiching the chaos with human meat on both sides. It's a recipe for disaster regardless of what vector the force of the impact comes from -- and that's ignoring the batteries.
Please folks , when you drive a vehicle the heavy stuff gets strapped behind a bulkhead at the front or rear of the car, or atop the car with appropriate and well-tested strapping & mounting equipment.
Don't risk your life for the sake of convenience.