After watching the anime, I can say that the film is pretty much scene for scene taken from the first season. Some details were slightly changed, some scenes removed, which kind of takes away from the smartness of the story, but what can you do in just two hours. And it feels like it was actually adapted from the anime, not the manga, even if it adds some elements that are relevant only to the rest of the story.

That being said, what the hell were those wigs?! All the "children" were wearing wigs that just looked plastered on their heads. You could have just used normal hair, ffs! There is nothing wrong with "recasting" the hair.

Bottom line: same story, but a bit lackluster. I recommend just watching the first season of the anime and stop there.