To get the Hunger achievement in Little Nightmares 2 you'll need to repetitively use the vending machine that you can find just before you get sucked into the TV during chapter 3 (the hospital chapter). Here is my own video guide that shows how to get the Hunger achievement along with all of the other achievements Little Nightmares 2 has to offer. The Hunger achievement is explained at around the 12:05 minute mark during the video.