Once accused, a person had to decide whether to admit to or deny being a witch. If a person said he/she was a witch, he/she generally saved their secular life while damning their afterlife. If that persona was honest and denied associations with witchcraft, he/she protected their soul in the afterlife while risking their secular life. Since it was the young girls who were doing the accusing, it was the children who where suddenly and essentially in control of people's souls--thus, acting as the gatekeepers to the kingdom of Heaven.
This line is spoken by John Proctor in Act II, Scene 4. At this point, Cheever and Herrick have come to take Elizabeth Proctor away because of Abigail's claim that Elizabeth has bewitched her using the poppet.
The point of the quote is that Abigail and her gang of friends (the little crazy children) are in control of all of Salem. People who were otherwise well respected can now be brought in front of the Court, and eventually executed on the basis of the testimony of these young people.
So, the phrase "jangling the keys of the kingdom" refers to the idea that they are in control. And the "crazy little children" part of the quote refers to the idea that they are just young people acting in a way that makes no sense.