I saw it today and REALLY loved it, so did my DS-10, DS-4, and DD-7. I am sure the rating was for the fightong scenes and the skeleton factor. I would judge what is right for your kids, but if they would be interested in a pirate movie anyway, they can probably handle this movie and everything in it.
edited to add-
Although he is completely without morals and willing to do anything to get his way, Jack Sparrow is played as a romantic hero. How do moviemakers win audience support for him? Considering the reality of damage and death he caused, what would be a just punishment for him?
this was from a website wher parents grade the movies and it is HIGHLY inaccurate. I would say if you have REAL doubts, see the movie yourself first. I have noticed LOTS of critics of MANY movies see things their own way (this site was in shock because Depp was in a Disney movie

) In my opinion they all have their personal opinions, as you will too. And of course this same website promotes vidoe alternatives, Muppet Treasure Island (realism?)and the Lord of the Rings 1 and 2 (what about THAT violence?)
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I like this film because it never made pirates out to be the goods guys like a lot of them do. Jack Sparrow was bad, but he was true to his word. Will remained a blacksmith rather than choosing a pirate's life. Elizabeth saw pirates for what they where really like and probably lost her fascination with them and accepted Will even with his "pirates" blood.
As you can tell, I liked it and the messages it sent.