Ironically, it did not go as smoothly as I hoped when we saw it on Friday. It wasn't bad or anything and our girl did pretty well, but she just wasn't taken with the movie (although she asked to got to the movies again when we drove by the theater yesterday). The wife thinks it could be because...
If you are concerned about the loudness, you could always bring a SPL meter to check the levels. I've considered it before. I mean you've got to have one right? Isn't it a requirement for posting on the Home Theater Forum? :)
I think she is old enough and we are planning on taking our daughter to see it, but it won't be her first. She does well at sitting and watching without too much restlessness. Ratatouille was an exception (in retrospect she was a bit too young at the time and it is not the best of the Pixar...