A ride being "scary" is so subjective that it is impossible to say what will frighten your particular child. I remember when DS11 was 2, he rode all sorts of rides with no problems and only got scared on one ride, Winnie the Pooh! Go figure!
Honestly, about 95% of the attractions have the potential to be scary to someone depending on what someone is afraid of. Your best bet is to get a book as PP mentioned and check out all the descriptions. Another suggestion is to have someone in your party check it out first and see what they think.
If "spooky" is your concern (not heights, noises etc...) then I'd say some rides to exercise caution with are:
MK - Haunted Mansion, Pirates of the Car., Snow White, Stitch
Epcot - Maelstrom
DHS - Tower of Terror, Rock 'n' Roller Coaster
AK - Dinosaur, Expedition Everest because of the Yeti, It's Tough to be a Bug