I can't recommend enough to watch Netflix's Hilda series! It's charming, intelligent, engaging, entertaining and positive, fit for any age category. Hilda and the Mountain King is not really a separate film, but a continuation of the two seasons of the show, so watching it without watching the series doesn't make a lot of sense.
Hilda is again put in a situation where established custom falls short and she has to figure out how to get to the truth and solve the problem, this time one that would affect everybody in Trollberg. It was a bit disappointing that the trigger to the entire film, the swap, was not actually explained. But other than that, the film was just as good as the series and probably just a rework of whatever they had done for season 3.
It's so sad that Netflix's strategy to get more viewers and money does not extend to caring for them once they are in. The system, as is, motivates them to make a lot of shows, but not to continue them. If a viewer felt for it and subscribed, that's great, it doesn't bring more money to continue the show that got them there. And that is probably what is happening with Hilda, too. I really really want to see the show continue, but at this rate, you should probably consider this film as a "wrap up".