This question i feel is something that no one will ever know, I don't even think Barrie knew what it was, but isn't that part of the fascinating mystery of Peter Pan.
My idea is that every child has a Neverland in their dreams and that every child designs their neverland to their own specifics. Neverland is a blank canvas before you are born, when you are and you are free to dream, neverland is in your image.

Thats it really