First horror movie for a kid

My niece (8 years old) likes to watch scary videos on youtube. I'm talking jump scares with creepy faces, monsters chasing people etc. Its not at all childish and some are generally creepy. (I like them, lol) So now she wants to watch a 'real' horror movie with me. Her mom gave approval, but she doesnt like horror herself so she cant decide what movie is appropriate and she trusts that I, as a big horror fan, wont traumatize her kid. My niece mentioned IT, she saw a short clip from the 2017 version and realy wants to see the movie. I am a bit torn about it. I think she will like it, it might scare her a little but that is, ofcourse, what horror is supposed to do. But I'm also wondering if it might be a bit to much for an 8 year olds first horror movie. I saw my first horror movie when I was 6. My older brother was babysitting and decided to watch Alien with me. I had nightmares for a week (and my brother was grounded for that same week, lol). However, the nightmares faded and my interest grew. I wanted to see more and this is exactly where my love for scary movies began. I want her to have this experience too, the thrill of being scared shitless but also the growing love for scary things. I'm just wondering, fellow scary movie lovers, what are your thoughts on this? How old were you when you watched your first horror movie and how did you react?