Just because Room is told from the perspective of a child, that doesn't make it childish. Jack talks like a five-year-old would: he personifies inanimate objects and cutely exaggerates everything he has to do. This is how he describes his day: "We have thousands of things to do every morning, like give Plant a cup of water in Sink for no spilling, then put her back on her saucer on Dresser. Plant used to live on Table but God's face burned a leaf of her off" (1.80).
Through his voice, Jack boils down complex philosophical ideas into statements that even we can almost understand. For example, at the end, when he and Ma return to Room, Jack says, "Maybe it's not Room if Door's open" (5.1112). He may not have grasped the use of articles left, but he understands that Room isn't the same when the Outside is accessible from it. Before, Room was set apart. Now, Jack and Ma are a part of everything. And the world might just be better because of that.
How would this story be different if Ma narrated it instead of Jack? Because Jack himself does not fully understand what is happeneing, a lot of the horror of the story is left to the reader's imagination. Does Jack's child-like perspective make the horrific events of the novel easier to read, or does it make the events seem even more horrible?