The major difference is in Tarsier's borrowing from survival horror, with stealth and hiding playing a key part. Get up close to Little Nightmares, however, and its rooms and corridors are constructed using entirely different techniques to those used by Playdead. There are many similarities, not only in the macabre tone (both contain a lot of child death and an oppressive atmosphere neither contains buckets of gore), but in the side-scrolling puzzling and occasional platforming. I'm going to get the Inside comparisons out of the way now so nobody is waiting for any revelations on that front. It's a horror game but it's mostly bloodless and doesn't rely on jumpscares or sudden shocks. Little Nightmares is the story of a little girl in a horrible place.
