It's one of those rare films that wouldn't work as a book, because its central idea is only cool when you're looking at it. Patema is an reverse-gravity girl. She and her people fall upwards and walk on the ceiling, although from her point of view it's us who fall upwards.
In prose, I don't think that would have much impact. On a screen and beautifully animated, though, it might do your head in. Well, a little bit. You'll have Patema (upside-down) and Eiji (normal) sharing the screen, interacting and even grabbing each other to neutralise each other's weight. You'll have the camera turning upside-down to show the viewpoint of the other person in the scene, which is an interesting exercise in subjectivity. You'll have people falling upwards into a huge, beautiful, blue sky. You'll have anti-gravity cylinders and backpacks, because everything from Patema's world obeys the same physics.
I'm not sure they're being completely consistent about how this works, mind you. Who's heavier, Patema or Eiji? Should they fall quickly or slowly? However it's still fun to think about and discuss. Since Patema is capable of eating Eiji-gravity food, for instance, extreme care would be required on the toilet after drinking Eiji-gravity water.
The plot, though, is simple. The film's capable of getting so carried away with impossible visuals and things you'd never imagined that it forgets to take us inside the characters. My attention sometimes wandered. It is, though, basically a good film. The plot's good enough and there's a lot of charm. Eiji and Patema are very likeable and there's a very hissable villain.
Patema's world is underground. She and her people are diggers and scavengers. You could call them troglodytes if you really wanted, but they're nice people.
Eiji's world, on the other hand, is a fanatical semi-religious dictatorship. You get on a moving pavement to be taken to the correct upbringing process for a correct education. Security cameras are everywhere. You always have to look down. (Looking up at the sky will earn you demerit points in class.) You'll be taught that your people are the only true humans and that you live in the only world. Once upon a time, scientists learned how to generate power from gravity. This went wrong and almost everything fell into the sky, so now your world's dictator teaches that going into the sky makes you an evil sinner.
Four OVAs were released the year before, incidentally, called Patema Inverted: Beginning of the Day. I watched the first three, but they're just six-minute chunks of the start of the film.
What would children think of this film? It's got a PG certificate, but I can imagine tiny children either having their minds blown or else finding its narrative too abstract. Well, try it and see.
This is a nice and mildly mind-bending film. The dictatorship and its security police are heavy-duty baddies, especially when our heroes are just two children. (I loved the moment when they make everyone look up.) There's some sad backstory, but also optimism about the future. I'm glad I saw it.