Teenage mild horror, full of plot contrivances
This is a bit more than fantasy, there are quite a few bits that would qualify as mild horror, and some action scenes had me on the edge of my seat, which may be what teens are after, but would be too intense for smaller children.
The story itself has some good elements, but the plot is contrived in many aspects. The hobgoblin is probably the biggest contrivance in this movie, solving "annoying" problems for the film-makers, such as giving "sight" to all the kids so they don't have to find more seer stones, and eventually killing the big baddy in one gulp.
Another contrived part is the splitting up of the twins from each other, which occurs during the majority of the film. The parts where they are together are brilliantly done, but it is fairly obvious that it is technically very demanding, and was kept to a minimal.
The movie is quite full of such convenient twists that do not add to the story but merely seem to be there to avoid major inconsistencies or technical problems.
Otherwise, quite entertaining, although very predictable :)