Segway? A sort of scooter that maintains its own balance. This is done using a computer and sensors for acceleration, gravity etc. It has two tilt sensors and 5 gyros according to the first link. The computer is part of a control system which calculates the power to apply to stay upright, and changes this to respond to the driver's balance and other needs like turning and speed.
The mechanical power represented by 55rpm and 3N.m is:
Power_W = (torque_N.m x 2pi x RPM) / 60
= (3 * 6.28 * 55) / 60
= 17.27W
The wattage mentioned is the mechanical power. It is the maximum power that can be delivered. The amount of power depends on the mechanical load, so this means any power up to 17W can be driven. This is not a lot of power, it is going to be a small model with light loads.
The electrical power when producing this mechanical power might be 12V @ 4A = 48W, assuming the ratings are at 12V. If it is 6V then 24W. It is just V * I at a particular mechanical load. The low efficiency is partly losses in the gearbox. The electrical power to the motor probably doesn't change much even when there is no mechanical load, in this situation.
The mechanical power is used for several things. The total power needed is the sum of all the powers needed at the same time:
Acceleration, using force = mass x acceleration
The force comes from the torque applied to the road by the wheel. If there is 3N.m maximum torque, and the wheel is 0.1m radius, the maximum force is 30 newtons. One newton accelerates 1kg at the rate of 1m/s/s. (every second the speed increases by 1 meter per second).
The acceleration of gravity is 9.8m/s/s, so to maintain balance presumably that acceleration has to be overcome at some times. This depends on the angles involved, so a bit of trigonometry would come in.
To climb hills, power is needed as:
Power_W = (Mass_kg x Gravity_m/s/s x Height_m) / time_s
To overcome friction losses power is needed. Consider friction as a force pushing against the desired direction. This is a relatively unknown number more likely to be measured in your situation. At a steady speed and not climbing, there is only friction, and if less than 17W, the vehicle will eventually get to a speed where the wheels are turning at 55rpm. The circumference of the wheels (2*pi*r) then give the speed. The speed can be controlled by reducing the voltage to the motor, but the power is also reduced.
I think this motor is much too small, even if it is for a kid. Look up 'balancing robot' for more about small things that do this. It needs 2 motors of course.