I would be hesitant using those hinges. I wouldn't trust them with the weight of my body, bike, the ramp, and the kinetic energy from rolling onto the ramp, on the little pivot points on those hinges. They may last awhile, but since this is at a park, with varying weights, speeds, and abilities, the chance shouldn't be taken in my opinion.
G2g now, but I'll post how I made my teeter-totter pivot later.
That was stupid to a felony level.
Give me eyes that are sharp, a will that's infallible, to perceive accurately, and react quickly.