To unlock the Wild Kids achievement in Little Nightmares 2 you'll need to collect all of the glitched remains during Chapter 1. There are 4 in total, here is my own video guide that shows all of the achievements/collectibles available in Little Nightmares 2, the timestamps for chapter 1 and all of the other chapters can be found below the video link.
Chapter 1Glitched Remains #1 - 0:23
Glitched Remains #2 - 0:47
Glitched Remains #3 - 4:51
Glitched Remains #4 - 6:35
Chapter 2Glitched Remains #1 - 6:56
Glitched Remains #2 - 7:18
Glitched Remains #3 - 8:34
Glitched Remains #4 - 10:24
Glitched Remains #5 - 10:41
Glitched Remains #6 - 11:26
Chapter 3Glitched Remains #1 - 11:45
Glitched Remains #2 - 14:02
Glitched Remains #3 - 16:07
Glitched Remains #4 - 16:15
Chapter 4Glitched Remains #1 - 18:42
Glitched Remains #2 - 19:22
Glitched Remains #3 - 19:44
Glitched Remains #4 - 20:25