Nothing extreme doesn't preclude an accident, that's why they are called accidents.
Sorry the difference between what you describe by tying a knot in the belt, versus using the original factory setup is:
1. The original factory setup is a tensioned system, that can tighten in an instant. A knot on the other hand, when faced with the forces of a crash, will stretch. This means your child is going to go further before the belts "catch", meaning a highly increased risk of physical harm.
2. The knot, no matter how tight, is not the proper way to stop a belt from moving. Event the center seat is not tied on to the frame at the end, it is bolted securely.
It is up to you, but I wasn't talking the danger on the street. Things can happen on a trail so fast, that even "putting around" is a huge risk to your children. Some people unfortunately learn the hard way, I hope you will take the advice that both 4x4high and myself have given you, and do it the right way.