Continuity mistake: The clock on the bomb's secondary timer doesn't match with reality while Prentiss is trying to defuse it - going from 15 seconds to 10 too slowly, then skipping the final few seconds so it can end on 1 second remaining.
Factual error: While JJ and Will might have exchanged "commitment vows" with a willing officiant, they could not have actually married in the time frame given for multiple reasons. The bank robbery occurred on a Saturday and the proposal was that evening at the hospital, with a stated intention to go down to the court Monday to wed. Instead, Rossi and Will set it up for the next evening, which was Sunday. The following is taken from Washington, D.C.'s marriage information site: 1) the court offices that issue licenses are only open M-F; 2) there is a mandated three day waiting period between obtaining the license and performing the ceremony; 3) there is a ten advance notice required to arrange a civil ceremony through the court office. Therefore, the earliest they could be legally wed would be on Thursday with a license and a private ceremony, while a civil ceremony would actually have taken an additional week. As for JJ's "lets get married tonight", I can't see any hospital chaplain obliging.
Answer: "The Boogeyman", s02e06. Spoiler alert: The kid is Jeffrey Charles and at that point he had already killed 3 other kids. Jefferey basically hated and resented other kids after his mother abandoned him and his father spent a lot of time with other children because of his job. Jeffery would lure kids Into the Woods and then beat them to death with his bat just because of hate, no particular "reason." The scene you're describing, Jefferey agrees to walk Tracy home when her "buddy" leaves her (at this point the FBI announced there's a serial killer on the loose and kids should walk with a buddy). Jeffrey gets mad at Tracy for walking too slow and for complaining, and he takes a swing at her, causing her to flee Into the Woods and have Jeffery chase her.
Bishop73