Won't Get Fooled Again - S1-E3
Other mistake: Just after they were finished explaining that you can not tilt the mystery package because it would instantly blow up, they show the little girl at her doorstep holding the package completely tilted. Just when you think that means that this package now doesn't fall into the "explosive" category, they then show the mom freaking out, and later police making the girl hold the package in a completely different (non-tilted) position.
Other mistake: When Morgan is showing the rest of the team the drivers license photos of the hostages on the train, every one of them is wearing the same shirt as they are on the train.
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