I think The Wizard of Oz is pretty scary for young children, with the green-faced witch and flying monkeys. My grandchildren have never seen it (11,9,7). The boys won't want to see it I'm sure, but but maybe the girl might.
What a fantastic film Fantasia was because there was something for everyone, and it gave children a brilliant introduction to classical music. Although there were two dark sequences. Night on Bald Mountain really scared me, when the demon came out of the mountain and all the graves opened up. I always saw him as the Devil from then on. Then I cried during the Rite of Spring when the dinosaurs perished in the desert, just bones and carcasses left. Their suffering was so palpable, it made me so sad, more than Bambi's mother even.