Disclaimer right off!!! I'm not an electrical engineer. Back in the 80's when power wheels came out I bought them and of course I modified them for longer run time,more speed and more torque.Thru this process I learned A LOT about electric motors.Your right there is no controller on them- That's how they get away from the expense. small low volt battery with a fuse tied directly to it,LIGHT WEIGHT,tires that have very little traction.I burn't motor after motor up because I bought many extra batteries and kept them charged so play time could be extended only to find out that when running the motors that long never gave them a chance to cool down and Thermal run-away occurred due to overheating.I had to make a rectifier to eliminate/minimize this issue. Your kart is HEAVY and the principles needed to power it change,also the fact that you have tires that will grip and a frame that won't flex contribute to the expense or need for a controller.Power Wheels makes a great product and have all the components well calculated for short use. The story about the car starter being used can happen very simply- run time would be very short as well due to no way to cool the armature.I'm not trying to discourage you,it's ALMOST simple to make an electric kart.The controller provides acurate milliamp feed thru diodes or shunts to keep the motor running cooler and eliminate thermal-runaway.Sorry so lenghty- Does this help you understand the science any better.
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