The Wee Free Men is told from an unnamed narrator's point of view, but we still get to see into Tiffany's thinking. We only see what she considers though, and nothing from the perspectives of other characters. As readers, we follow Tiffany's thinking in real-time, which is extra important since one of her main gifts is her ability to think critically. For instance when Tiffany tries to figure out where the doorway to the fairy world is, we get a front row seat into her thoughts:
But nothing should happen, she thought. You can't leave a doorway into your world that anyone can walk through, otherwise people would wander in and out by accident. You'd have to know it was there. (7.370)
Tiffany is savvy enough to figure out that the doorway won't be obvious and that she'll have to work a little to find it—and because the narrator has access to her thoughts, we learn all of this too. It helps us understand both the fantasy lands we're hanging out in and the young girl we're following around through them, which keeps us hooked as we read.