Thanks for all your suggestions. In my searches I had already come across "The Wonderful Flight to the Mushroom Planet", but I don't think that's the one. The story description of "The Spaceship Under the Apple Tree" sounds closer, but I don't think it's that one either. For one, there doesn't seem to be a Dutch translation, and I'm almost sure I read it in Dutch.
As for the other story, the description of the Outer Limits episode seems quite different from what I remember, so that's not the direction I should be thinking in. For example, I don't remember there being any spaceship in that story. But then, I don't remember a lot of things that actually were in the story... But I remember it as more of an adventure story with traveling around to world to find all these children (I think there were four or five or so) for some magical purpose or so. But it played in a somewhat modern setting I think, not a medieval 'fantasy' setting.