It's easy to understand why third-graders might not refuse to obey their teacher, but when the exercise is done with the prison guards, why don't any of the adults object?

The children have to fear for they parents, and just want to be accepted. Adults have their own vision of the world, and if something goes agains that, it is likely that they do not obey.

Explanation:

It has a lot to do with reputation, and fear of your parents for the children and reputation for the adults.

For example, in the third grade, if you do not obey your teacher, your parents are likely to be called, and they may punish you. As a children you just want to be accepted and avoid problems, so you obey.

In the prison you're now an adult, with your opinions and vision of the world, not worried about being accepted and avoid problems. So if something happens that you do not agree, you end up not backing down, that is, not obeying the prison guards.