[Chaychay]

A couple years back FIRST provided current sensors in the kit. If you put one of these on your Fisher Price Motor circuit, you could just tone down your PWM output as the sensor output got closer to 40 amps. (Like a proportional controller)

EDIT: I just read the posts above mine, and they render my post incorrect. But just for your reference, you can stick a current sensor on their to monitor how much you are drawing.