Electric scooters are a good thing to look at when building a light duty kart. Most of the parts will carry right over to a 4 wheel kart. I, for example, started with a 250w 24v scooter, hacked off the back half, and welded it the front half of the "itzy bitzy" frame for which free plans are available. I ended up with a reverse trike and I didn't have to design the motor, brakes, etc since it was already done. If you want reverse, you'll need belt drive as it'll allow the wheel to turn fwd and reverse as opposed to a bicycle style freewheel. a fwd/rev switch from a power wheel will easily carry 30a at 24 but make sure he doesn't touch it while moving or goodbye controller.
Since you'll probably be using mechanical brakes, you'll want a way to shut off the controller if brakes are applied while throttle is also being applied. Scooters have a special brake lever for this with a switch but I'm not using a scooter controller. I used a spst relay with one coil wire going right to the neg terminal on battery and the other to a throttle button, then to the brake lever button (drilled hole in brake lever and inserted a small momentary switch tthat's pressed by the lever). For the relay to switch on, the throttle butted is pressed and the foot is off the brake. Braking will break the circuit going to the relay and turn off the controller. II'mgoing to do a write up soon if that ddidn't make sense...
