"'We need a place,' she said, 'just for us.'"(p.58)
Recently I read a book. A scriptwriter said in it, "There should be some places where children can escape."
The leading characters of Bridge to Terabithia are two 5th grade children. They can't get used to something standard. It might be hard to define what is standard. But there, especially in school life, are what it should be or who they should be.
Terabithia is the place which soothes them. Something very sad occurred and changed the boy and the girl. After that tragedy the relationship between Terabithia and him has also changed.
At the end of the story he built a thing. Building such kind of thing implies that Terabithia is no more for just two of them. He may have prepared a place which someone else can escape into.