Throughtout the past few years, extrinsic motivation has been approached from two different perspectives: on the one hand, it has been used by parents and teachers with the only purpose of "controlling" their children and on the other hand, it was regarded as entirely useless, since the student must come to learn just "for the pleasure of it".
Both perspectives are probably correct as long as we approach them from the right angle. It is not advisable for a parent to condition learning by an extrinsic motivator ("I will buy you X if you get an A). For that matter, research has shown the inefficiency of such a perspective in various areas (not only the educational one).
Many a time, external rewards earned by an individual tend to weaken his personal interests, his joy and his satisfaction, since the person in question does something for purposes other than the thing itself (Huffman, Vernoy, Williams and Vernoy). For instance, the companies that offer substantial discounts on purchasing their products during a promotional campaign (extrinsic motivation) sometimes notice that after withdrawing the promotion, many buyers who had purchased their product at a lower price do not remain loyal to that product.
We were stating though that the truth is much more complex than that. The specialized literature, through the perspective of several researchers (Ryan Deci on the one hand and Manderlink Harackiewicz on the other hand) pointed up that external rewards do not invariably reduce the intrinsic interest for an activity. A positive extrinsic motivation occurs when the reward is based on competence and comes after achieving an outstanding performance; in this case, the reward is not a prize for simply engaging in an activity. A reward for competence produces a strong feeling of pride and can support the individual in his desire to have a similar future performance.
Furthermore, as noted by another researcher (Carlson), many of the reinforcements / rewards we receive are not direct, but they are related to our behavior. Thus, if a child tells a story properly, he can be extrinsically motivated by receiving a candy. However, a form of positive external motivation can indeed occur in this activity (although it can no longer fall so easily into extrinsic motivation) if the child is rewarded based on his audience's interest in his story.
To conclude, although a "candy" is easy to buy, it does not have such a high efficiency. Not even a whole bag of candies or the bike most wanted by your child. However, if you show interest in what he does, praise him for his success and show joy whenever he reaches a success (even a small one), then there are chances for you to be on the right track.