This one is in the section right after Six is kidnapped by the bully enemies. The school building is a big "L" shape head to the juncture (past the stairs) towards a dumpster, and you'll find the remains. Number 6 is found on the playground, just before you climb the rope into the school. After leaping across, climb the stairs and go through the door on the left. You'll need her help to jump across a gap. In the first section, you'll push a television off a ledge and Six will ride up a rope to a level above you. This collectible is technically in the School region, but you find it before you reach the school.
After you ride across the water and get to the beach, run up the beach towards the boxes in the background and you'll spot it. This one is at the very end of the Wilderness section. Approach this and Six will offer to boost you up to the handle.
After you leave the first building with Six but before you enter the hunter's shed, you'll see a little outhouse in the background. Duck back under the log after sliding through, and you'll find another tunnel with the remains at the end. This one is found before the section with the bear traps covered in leaves, right after you slide down the hollow log. Approach the screen, crouch, and move right to find it by a shovel. Drop down like you normally would, but don't climb up the other side.
The first is found when you come across a broken bridge. If you want to see everything Little Nightmares 2 has to offer, here is where you can find them for each section of the game. They look like shadows or little ghostly children, and touching them will cause Mono to clutch his head and twitch around about. Some of the collectibles are out in the open and easy to find, but others are squirreled away and easy to miss.