I remember seeing it in the theater w/ family and my (then) four year-old niece saying "this movie is really sad". The first 20 minutes are about how the old man lost his best friend and wife and is left all alone.

It wasn't sad in the same sense that Toy Story 3 was sad, because I don't think little kids can really relate to Toy Story 3 story like adults can. Kids could realize, at least on some level, the loss felt by the old guy in Up.

Easy to miss that it was rated PG... I think a lot of parents took little kids on the basis that it was animated and pixar alone.