Electric Go Kart
Use the wheels, the motor, the transmission and the battery as the core components of an electric go kart. You will have to design and weld up a frame out of steel tubing used for bicycles. If you do not have a welder, most welding shops can do this for a reasonable fee. For a low-speed vehicle, a go kart can even be made out of wood, with plans readily available. You will have to use a little ingenuity to modify the plans slightly for a motor and battery, but this should not be too difficult.
Electric Tricycle
You can design and build an electric tricycle. The "trike" can have a low rider chopper look, or a more traditional child's tricycle look. You will have to design the frame, using steel bicycle tubing. Put the motor control on the hand grip, so a child can actually twist the control to move the trike forward.
ATV
An ATV, or "quad-runner," can be built using the motor, battery and controls. Obtain a large tricycle. Use the back part of the frame and develop two wheels for the front instead of one. With a little ingenuity, you can make a very good looking quad runner for your child.
Four-Wheeled Scooter
A four-wheeled scooter looks like a scooter, except it has four wheels. The child stands up on a platform and a long handlebar provides the steering. You will have to become a design engineer to figure out the dimensions. Place the control unit on the handlebar, so the child can twist the hand grip to make it go.