I have the image of the injured bird in "Up" limping along burned in my memory. I actually thought the scene was a little mature for the kids watching the movie, so I agree with you about them convincingly conveying emotion.
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.