To unlock the Sick Kids achievement you'll need to collect all of the Glitched Remains in Chapter 3 (the hospital chapter. Here is my own video guide that show how to get all of the Glitched Remains that are in Little Nightmares 2. If you're looking specifically for the Remains in chapter 3 you can find the timestamps just below the video in this post.
Chapter 1
Glitched Remains #1 - 0:23
Glitched Remains #2 - 0:47
Glitched Remains #3 - 4:51
Glitched Remains #4 - 6:35
Chapter 2
Glitched 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 3
Glitched Remains #1 - 11:45
Glitched Remains #2 - 14:02
Glitched Remains #3 - 16:07
Glitched Remains #4 - 16:15
Chapter 4
Glitched Remains #1 - 18:42
Glitched Remains #2 - 19:22
Glitched Remains #3 - 19:44
Glitched Remains #4 - 20:25