This might sound dick, but I have a 2 year old and a four year who practically live in the steps, it's like God's gift to them or something, they are obsessed. Could nstantly opening and closing the doors, my four year old opens it while my 2 year old climbs, still never fallen. I have a three step set of steps and they have never fallen once. If you have six and seven year olds falling, you might have bigger issues than fixing the steps.
After a few times of them letting the cold air out they quickly.learned what happens punishment wise, and magically they've pretty muched stopped opening the door. Maybe you should try that approach, telling them to stop?