It’s because those quite plausible possibilities mean something different in a kids movie. Some of the same can be said about Ugo Bienvenu’s “Arco,” a charming and dreamy sci-fi animated movie where environmental catastrophe and cartoony fun collide. “Arco” is the unusual movie to exist in two future times, never our present. Iris, a sharp young girl, sees Arco’s rainbow fly into the woods and runs to find him. In “Arco,” kids are left to fend for themselves in a world of technology and ecological disaster.