Don't Go (2018) on Netflix

Plot summary

Ben and Hazel Slater are grieving Molly, their only child, after her death in an accident. They try to start again by renovating a hotel by the sea on Ireland's west coast, but the building is full of memories. Ben keeps seeing the words Seas the day, and he remembers a summer day on the beach nearby when the family built sand castles and crowned them with small red flags. He starts to wonder whether the words carry a message for him. When he wakes from one of his dreams holding a tiny red flag, he becomes certain that he can reach into the dream and bring Molly back. The people around him begin to question his sanity.