Obviously, The Never Ending Story is one of my all-time favourite movies.
I wonder if you agree, that child actors seem to be able to act best, when playing roles in fantasy movies? Characters that are not limited by everyday boring life, and allow them to develop their own spin on the character.
The "real life" aspect for me, really depends on how good the script/screen writer really is, and being most likely adult, can they really see the world from a child's prespective? I'd say, even for a professional writer, being able to reproduce a realistic childs view, is quite difficult. Indeed, some have managed to do a good job, but perhaps they tapped into their own childhood experiences.
Whereas, a story involving a "fantasy" idea, has no limits. There is no real, exact way, that the actor should be or act. They can make the role, their own.
Noah made that role, he was and is, Atreyu. Same for Barret. No remake of that film could really replace them, at least not from my point of view. They were and still are, those characters.
But with a movie about an everyday "real life" family, the child actor could easily be replaced by another in a future remake.