You also need to create a higher voltage out of the motor than you already have in the batteries.
On DC motors, the higher the voltage, the faster it makes the motor spin.
I assume that you were pushing the PowerWheels faster than the Powerwheels itself could drive right then. Those little toy cars typically use a pair of very small permanent magnet motors.
When she would press the "GO" button, it completes the circuit between the motor and batteries. The drag of the magnets would increase resistance on pushing the car because you are now making a magnetic field in the motors and pushing a little power back into the batteries.
On
Electric Vehicle Racing , when they were racing a car on the salt flats, it had a sepex motor in it. To recharge the car, they would drag it behind a pickup truck with the car in gear to regenerate and charge the battery pack!