That makes no sense. If you decouple the crank arms, you screw up the mechanical action needed to actuate the drive train. Training wheels are often just a bit off the ground to train balance: experienced cyclists can ride in a straight line without the wheels ever hitting the ground; but they prevent you from learning to lean in a turn or curve, or really to steer properly at all. Training cranks? I can't imagine the usefulness, since (unless you're clipped in) your resting foot follows anyway, and breaking this link removes mechanical synchronization that is critical to learn.

You can't ride a bike by flailing your legs all over the place.