Several of the movies suggested are great but my dd still found them scary as a preschooler. In several, The Fox and the Hound, Dumbo, Bambi, and Nemo the Mother is killed or removed very early on which many little children find very upsetting and confusing. The Fox and the Hound also features a hunting theme that goes on throughout the movie and a pretty scary attack scene with a bear(? I think...)
My daughter absolutely loved Lady and the Tramp and Pooh Bear at around that age. Lady and the Tramp has a couple of scary scenes, a dog fight and a scene with a rat in the house which the Tramp then kills. Pooh Bear has no scary or distressing scenes. I also really like Robin Hood for that age group. I know it isn't "old school", but my daughter also really enjoyed Cars. The greater plot points may have gone over her head but the movie is visually arresting with lots of great songs and fun racing scenes and no upsetting or scary content whatsoever. It is long though, just fyi. I also have a soft place in my heart for Pocahontas which has beautiful old fashioned hand drawn animation and some lovely songs. The central conflict is about respect for one another and for the earth. It gets overlooked but its very nice.
We also got a couple of DVDS of old Disney shorts and we would sometimes watch just one or two little cartoons, they mostly run under 20 minutes and each DVD has a selection of different kinds of movies. For example one DVD might have some of the old goofy cartoons, on one we have "The brave little tailor" which is very cute with Mickey Mouse, maybe a Donald duck cartoon, etc... Also, Silly Symphony cartoons are a series of sweet shorts set to classical music themes. You can youtube some of those which makes them super easy too.