To unlock the Pale Kids achievement in Little Nightmares 2 you need to collect the 4 Glitched Remains that can be found throughout chapter 4. Here is my own video guide that shows the locations of all of the Glitched Remains including all of the other achievements Little Nightmares 2 has to offer. Timestamps for the Glitched remains 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