Let the Right One In (2008) on Netflix

In the suburb of Blackeberg in Stockholm, the twelve year-old Oskar is a lonely and outcast boy bullied in school by Conny and two other classroom mates; at home, Oskar dreams on revenging the trio of bullies. He befriends his twelve year-old next door neighbor Eli that only appears during the night in the playground of their building. Meanwhile, Eli's father is a serial-killer who drains the blood of his victims to supply Eli, so she doesn't have to kill. Eli advises Oskar to react to Conny fighting back; however, he soon discovers that she is a vampire and he feels fear and love for the girl.