Mika (my 8 yo) wanted to go see Percy Jackson and the Olympians: The Lightning Thief and she wanted me to go with her. After reading some reviews I was reluctant - it was mostly portrayed as a pour cousin to Harry Potter with similar, but less developed, characters and plot. And indeed, that's what it was - but it was a well crafted adventure movie, with great special effects, approachable characters and it held our interest. And, to top it all, it also taught us a little bit about Greek mythology (though you have to be careful not to take anything they say too literally). Hopefully it can be enough to spark a kids' interest in the subject (Mika is already into ancient mythology).
Camila, my 5 yo, found some parts scary and many boring, but she was a trooper and behaved through all of it.
In all, I'd recommend the movie for kids 7yo and older and even for adults who just want some mindless fluff.