This is an amazing movie. I saw the 3D IMAX and it was well worth the extra money. I found it thoroughly beautiful and moving.
However, for kids, it can be challenging. It can be very slow-paced. Dramatically, it works, but it may tax the less patient. Then, there are scenes of peril that are intense and relentless. There are a few gruesome moments where dead bodies are shown, and a few quick f- and s- expletives. A non-believing character struggles to form a prayer -- something I found interesting though folks with a strong religious or atheist bent might take umbrage (not sure why, but strong religious or atheistic folks seem to pounce on opportunities to take umbrage).
All in all, I think mature 12 and up can handle this one. Frankly, it's so good -- visually stunning, scientifically accurate (with a few errors), and emotionally vivid -- I think kids SHOULD see it. It is so far beyond most multiplex pap that it's worth a few challenges. Sensitive young ones might be upset at some points, but if they make it through, the overall experience will be uplifting.