Operant conditioning begins early in childhood in one form or another. Parents choose to discipline their children in a multitude of ways, the most popular being "time out." I first began using time out when my twins were two years old. As a new mom, I was unsure that it was possible for it to influence their behavior, especially at such a young age. Then I realized that being consistent with it was the only way that they would learn. They needed to know that every time they behaved badly they would get one warning, usually something like, "I'm going to count and if I get to three...," and any negative behavior after that would warrant a time out. Time out meant that they would sit by themselves for one minute for every year that they were. So my two year olds were sitting for two minutes. The more consistent I was, the more there bad behavior waned. Now, at three years old, when they are doing something that they shouldn't be, which is still often, I rarely have to get past the number one before the behavior ceases. They have learned that there are consequences to their actions.
On another note, the twins' younger brother, who is just turning two, has been participating in time out since he was one, as my method of discipline. He quickly got the point when he saw his brothers sitting in chairs, waiting for a timer to go off. Now, all I have to do is mention the phrase and he is automatically telling me he is sorry. Granted, he probably doesn't grasp the true meaning of "I'm sorry," but he is slowly understanding that with every action there is a reaction, making my job a bit easier.
Attached is another student's experiment with operant conditioning involving her four year old daughter.