Up was much better than I was expecting from the trailers. My only criticism might be that it would be slow paced for some children, but they at least seem to enjoy the scenes with cute animals.
It was very emotional. The adults are likely to cry, but most young children won't be affected by it on the same level. One little girl sitting near me did start to cry and had to leave, but it was in a tense scene with dogs barking rather than one of the scenes that was more emotional for adults.