HOW TO TRAIN YOUR DRAGON is an excellent family film. In fact, it's an excellent film for everyone. I had my doubts at first, the opening 20 minutes or so are pretty weak, but stick with it. Once it becomes the story of a dragon warming up to a human, it becomes much more endearing.
The characters are pretty good and develop well throughout, and the animation is very good. A lot of nice details on hair and skin. The voice casting is pretty good for the major characters. Randy Thom's sound design is excellent as always. John Powell's score adds tremendous depth.
Perhaps the only thing missing was that extra layer for adults that the Pixar films always seem to have. But that's not a necessity for a film to be good. There's a lot in this film for kids, and plenty enough for adults to enjoy.