There was an animated (i think animated) movie where 2 children, I think a boy and a girl, are in a blank white world and they make things from their imagination. Initially they each make imaginative creations and have fun together but end up fighting and destroying each other’s stuff then become friends again. At the end of the movie they break the moon or sun or something and find its actually a window/observation room, and there are people or scientists behind it. I’m sure its a movie of some sort… or I dreamed it up. I remember it from my childhood so it would be before 2000 I’m pretty sure. I’m not too sure about the stuff behind the moon/sun. But there were definitely two children in a world where they could make things from their imagination.