One thing that I noticed was that despite all the kiddie trailers and marketing to children, a lot of the kids in the theater were actually pretty restless during the movie, particularly during some of the more poignant scenes.
After asking around for opinions, it seems like the movie appealed a lot more to an older audience, people in their late teens to mid-twenties, because we were able to pick up more on the lessons that it was trying to impart. One lesson was that some people can change such as Hiccup's father Stoick, while others like Drago are so set in their beliefs that they refuse to change. My sister-in-law picked up another one--one about forgiveness after a traumatic incident--despite the horror, it is important to look at the situation in a reasonable manner and forgive your loved ones for things that were out of their control.
Ultimately, despite the fact that the style of animation is associated with children's movies, I think this sequel is aimed towards the kids who saw the first How to Train Your Dragon in 2010, who would now be older teenagers four years later, instead of the current kids, who enjoy the fight scenes but might not pick up on the themes--of learning to accept responsibility despite feeling uncertain about your own abilities, and of deciding who you are versus what people tell you who you are--mature themes that young adults can relate to.
RANDOM NOTE: For character design, I always thought that it was weird how Hiccup gets this nice, modern layered hair while everyone else has long, viking hairstyles. Here, he's about twenty and his jaw line is more firm and less boyish, while the hair is more stylized--texturizing hairspray? Who knows. :P
I imagine that Valka's character is a bit controversial because she strays away from some tropes about motherhood and wifehood (that you must be selfless, and if you choose to commit to a career in dragon-training over childcare then you're a horrible human being), and this is another aspect of the movie doing something that appeals to the older audience. Broken but living families are something that many can relate to, that are a bit underrepresented in childhood films.