Healing the family. Ben is going to be the best big brother ever – he’s not. The Dad is absent. The daughter cannot speak. The mother’s loss has left a huge rift. At the end, they are all together (as are the Giant and the the Owl Witch). This could make it the actual goal, or the result of overcoming grief.
But, then who is the protagonist here?
The only place where this goes wonky for me now is that there is a secondary goal of “getting the selkie her coat” – this is going to save (essentially) all the sidhe in Ireland. It’s such a secondary thing for most of the movie – though it motivates the sidhe to help, and the owl witch to interfere – but then it overwhelms the ending. I think this might be because it’s a story for kids, and provides the most magical ending.
FWIW, the steps of the hero’s journey are so apparent to me in this movie that I believe the author was trying to map onto them. It would make a true storyform incidental or accidental though not impossible.