I took 3 kids- ages 4, 6, and 9. All in all everyone enjoyed it. Very funny! I'm not usually a Jim Carrey fan- I tend to think his humor can be a little over the top/annoying/abrasive but it worked for this movie and we really only got a controlled dose of it. Must be a good director. Movie does not follow book but still enjoyable in its own right. Somewhat predictable and idealistic but most kids movies are so that's not a big deal. Story moved along at a good pace, which is pretty good for a live-action kids movie.
There were some parts that were a bit "emotionally intense"- dealing with divorce, one of the penguin eggs doesn't hatch, Mr. Popper has "job drama" (gets fired), Mr. Popper's resents his dad for not being around when he was a kid and then dies at the beginning of the film.
These might be a little tough for younger kids who are emotionally sensitive. About 2/3 of the way through the movie my four year old niece crawled into my lap b/c she said she was "scared" meaning that the emotional plot points were making her nervous. She's a bit on the sensitive side so if your under-5 child is too just be prepared. She was laughing most of the movie though and was pretty happy in the end. No permanent emotional trauma at all. Though I can imagine, as some other people have noted, that it might be a little hard on the younger kids if your family is personally going through some of these issues. (However, I also suspect that it's pretty normal for a kid to at some point ask their divorced parents if they'll get back together. If this isn't the catalyst for that question, something else likely will be. If the wound is fresh, be cautious but I might argue that it would be a good way to start a much needed discussion with your child.)
All in all, everyone came out pretty happy. When asked in the end what they thought of the movie, all three kids answered positively. They seemed to focus more on the dancing penguins and turning your apartment into a winter wonderland than on the relationship issues.
"What your favorite part?"
Girl, Age 9 - "when Mr. popper taught the penguins to dance"
Boy, Age 6 "stinky was funny"
Girl, Age 4 "i liked when they were sliding through the party"