Katherine Paterson was a fan of CoN. In the book, she mentioned that Terabithia was a Narnia-like magical country, Leslie let Jess read her CoN collection so he could learn about how to live and rule a country such as Narnia, and even threw in Aslan's name during the Easter discussion between Maybelle, Jess, and Leslie.
But in all of that, Ms. Paterson mentioned that she thought she had made up the name Terabithia, forgetting that there was an island called Terabinthia in CoN. And to her own credit, Terabinthia is not in any way prominent, appearing in only one book, "Voyage" and even then, it was not a major port of call.
MrBob