This is such a difficult topic. Everyone's ideas are different and what it really comes down to is understanding the content and what is scary to your child.
I am an absolutely horror junkie, scream Queen if you will. I have loved the genre since I watched my first movie and it's something that has grown with me. I was one of the typical goth girls in high school, and although the clothing and outside fashion as altered a bit my love of horror across all mediums is unrelenting.
I have two 7 year old daughters and this year they are really getting into the scare factor of the holiday! I love it but gauging their horror intake is a bit tricky.
What you need to consider....
Age - Many articles and rating systems use this as a bench mark. I suggest through that idea out the window. It's all about the person and maturity, not how many candles you've blown out.
Review it - Never show them something you haven't watched. This way you know what's coming and can limit their exposure. I thought of a lot of good films based on scare factor but they had adult content.
Know the fear - What are they frightened of? Dogs? Storms?, Clowns, the dark? Start with movies that don't tap into their greatest fears.
Look at it from their perspective - Poltergeist may not be scary to you (new or old) but it holds a lot of major fear triggers, separated from parents, the deadly closet, storms and creepy trees, etc...
Currently we have watched The Are you afraid of the Dark? Series and Goosebumps. RL Stines The Haunting Hour is a great series for young kids wanting a scare. One of my girls was terrified of the Pumpkin Heads episodes. This show has the nice twist of open endings without resolution. The kind that allows the mind to keep wondering after bedtime.
Sleepy Hallow, Silver Bullet, and many of the Hammer horror films have been on our viewing platter.
A few of the family favourites we have watched over the past few years. These are the classic fun flicks.
Paranorman - This is our household favourite!
Ghostbusters 1 and 2 and the remake
House on Haunted Hill (1959)
Cat People (1942)