Do you mean at home or in the theater?
Just yesterday I let my kids watch "How the Grinch Stole Christmas" for the first time. They LOVED it. I thought the Grinch might be a little scary but they didn't seem to think so. The younger two enjoyed it much more than Charlie Brown Christmas, which I think was a little too subtle for them.
Just based on my own experience a full length feature might be two long for your kid. But if you want to try it I'd go with Totoro. There's the old version My Neighbor Totoro, but I think the dubbing is terrible. Go for the newer Disney version with Dakota Fanning and her sister. It's very gentle, super cute, not scary at all.
"Mommy, I need to put on my goggles, because I have too much energy."