Electric drive
Hi Daviddoria
If your wheels are about 10 cm in diameter, and want to go a little faster than walking (4 KM/Hr), something like 10 KPH, or 166 MPM the wheels must turn at 530 RPM.
A radiator fan motor, with the fan may turn at about 1000 to 1500 RPM taking 15 to 20 Amp at 12V, enough power to impulse your vehicle at the desired speed.
The easiest way to transmit the motor power reducing the speed (is only my opinion) is via a friction drive with a rubber wheel attached to the motor shaft, (use something like a shock absorber rubber mount). With a diameter roll of 3.5 cm. at 1500 RPM you will get 10KPH.
Mount the motor over the wheel, pressing against it with enough pressure to avoid slipping.
With a 12AH 12V battery, a gel cell is safer, and using it only for short periods (3 to 5 minutes) it will last for 5 or 6 rides (assumming 20A ), but anyway the battery life will be very short) .
To justify this post in a electronic forum I would add a motor speed control using a PWM circuit and a power MOSFET , and a electronic horn or siren (just in case).
Regards and good luck.