No reason in my mind for a 2nd brake pedal, unless the OP is teaching the kid in a parking garage.
Just find a big open space and work them into it - I'll assume they've seen you operate a car and watched your inputs ?
My 1st driving was about age 12 in Mom & Dad's '57 Sport Suburban, when we were visiting friends in a rural area.
Dad just had me run it around slowly at first, and then we hit some 2-lane roads.
Year or two after that, he'd bought a little car with a 4-speed, and had me run it around our suburban block soon as I had the clutch/throttle interaction down in the driveway.
Pretty much all he told me was not to wail away at things, but be smooth & purposeful & observant.
Nowadays, I'd put a kid in a high-performance driving school pretty soon, to learn control & where the limits are, given modern traffic...