Other mistake: When Prentiss and Morgan are trying to figure out how the victims were controlled, Prentiss phones Garcia to find a drug that would temporarily paralyze or subdue. Then just as we hear Garcia answering the call, there's a close-up of Garcia's computer screen and we see the picture of five suicide notes, and at this point there have been six victims. Then later, after the seventh victim is found, when Morgan holds the latest suicide note in the close-up, we can see it's the same note that's already been shown.
Revealing mistake: When the unsub is in the house with the mother and her toddler, one of the shots of the toddler is flipped, before he starts crying.
Character mistake: When Morgan looks up Charlie Luvet's car GPS map history, the last address reads "11257 Palmdale Drive," but when Morgan phones Hotchner with the address he says, "11257 Palm Drive."
Other mistake: When it shows Keri's license it says it expires in 2003, but the episode takes place way after that year, considering Katrina had already happened.
Other mistake: At the start, the soon-to-be victim gets off the A train at Spring Street (subway heads south, then Bklyn/Queens) and stands on the west-side platform, facing the southern subway exit, then the unsub comes up from behind him, shoots him and he falls toward the same way that the unsub walks - the southern exit. However, when Hotchner plays the NYPD's surveillance footage, the victim is facing the northern subway exit when the unsub comes up from behind him, shoots, and he falls toward the same way that the unsub walks - this time the northern subway exit. Interestingly, neither are flipped shots since we can see "Spring St" correctly. Later, when Prentiss reviews footage it's as it was shown originally.
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