firefighter rope and knots?

I think what you are looking for is a "Bight."

A bight is where the rope is looped back on itself to form a "U" shape.

That's an element of a knot. The other two elements are a "round turn," and a "loop." A loop is formed by making a bight, but continuing to pull the ends up until they are parallel with each other, extending from either side of the underside of the loop.

A round turn is essentially a loop that is inverted, so that the ends fall alongside the loop.

If you're looking for a specific knot that starts with "B" that is used in the fire service, it would be the "Bowline," which is used for self rescue, victim rescue and utility hoisting.

The only other knot that starts with a "B" in the fire service is the "Becket Bend." Used to join two ropes of different diameters.

I don't think many kids would be interested to learn about ropes and knots that in depth. Most just want to see the gear, put it on and run through trucks. Of course, to do that they have to suffer through a fire safety lecture first...

Link has pictures along with descriptions.

Good luck with your presentation.