Other mistake: While Hotchner and Reid are talking about Hollow Man and the Mill Creek Killer, Reid's going through the newspapers and when he figures out that the two killers have been talking to each other via the classifieds, there are two closeups of an ad's text. After Hotchner suggests using their relationship against them, Hotch says that Reid's fake ad will imitate Hollow Man's language patterns to send a message to the Mill Creek Killer. Then we hear Reid reading the fake ad they just placed in the classifieds, but the problem is it's the same ad that was shown in the closeups earlier, plus an additional line at the end - "here's my gift to you."
Continuity mistake: When the team visit the home of Marcie Mitchell, the older boy takes the baby to another room. The baby is crying, then as they are in shot walking through the door the baby is looking around and smiling, and as the door shuts we hear crying again. (00:11:20)
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 her 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