I took my 5 year old and a 6 year old friend of his and both of them LOVED it! I don't think I"ve stopped hearing about Hiccup or Toothless since we left the theater, almost 2 weeks ago!
Best of all, I even enjoyed it and was totally taken in by the characters. I would definitely recommend this to anyone!
I know that there were some concerns about it being scary and afterwards, neither of the kids I brought had any issues with what happened during the film. While the dragons did appear ferocious, there wasn't any "gore" and while the sentement of loss was there towards the end of the film, it is fairly quickly eliminated. I found out later that my son's friend has had issues with being afraid in movies and he loved it.