In A Child's Dream, there are 2 rooms that can be maneuvered into walking on the wall/ceiling so it could get confusing, but you can check the official interactive map here to see the full layout and position of the chest (of course, this 3D map is also available in-game). In total, there are 25 treasure chests in this area, including puzzles and Lordly Trashcans.
While doing the guides, the official interactive map didn't fully update so there 2 are pins that are not marked yet. So I edited the images and added two circles (#21 and #25).
All Treasure Chests in Penacony (A Child's Dream)
1. Behind the portable partition and in front of a pile of books.
2. At the end of corridors, behind an enemy (but you don't have to defeat it).
3. Complete the Dream Ticker puzzle to unlock the treasure chest.
4. In front of the bookshelf.
5. Beside the bubble bridge.
6. Defeat the Formidable Foe to unlock the treasure chest.
7. Once you enter the door you just came in, go down the stairs to your right. Complete the Jigsaw Puzzle to unlock the treasure chest.
8. In the corner towards your right once you walk through the bubble bridge.
9. At the edge of the location, near the floating boxes and blue lights on your left.
10. Defeat the Formidable Foe to unlock the treasure chest.
11. Just beside the bubble bridge.
12. Complete the Dream Jigsaw to unlock the treasure chest.
13. Defeat the Formidable Foe to unlock the treasure chest.
14. Interact with the trash can and choose the correct option (Option 3 > Option 3). If you choose the wrong one, you will fight the trash can, but will still get the reward if you defeat it.
15. In front of a bubble bridge, there's an enemy wandering near it.
16. Beside the mechanism that fixes the blocky road. There's a group of enemies in front of this.
17. Complete the Dream Jigsaw to unlock the treasure chest.
18. In front of the door.
19. Complete the Dream Jigsaw to unlock the treasure chest.
20. Complete the Dreamticker to unlock the treasure chest.
21. In the same room as the Dreamticker.
22. Complete the Dreamticker to unlock the treasure chest.
23. Defeat the Formidable Foe to unlock the treasure chest.
24. Interact with the trash can and choose the correct option (Option 2 > Option 3). If you choose the wrong one, you will fight the trash can, but will still get the reward if you defeat it.
25. Defeat the Trotter.
26. There's one chest in the middle of the room but everyone already have it since we need to open it during the Trailblaze Mission called Through a Glass Darkly.
