The original play on which FN was based - The Man who was Peter Pan, by Allan Knee also leaves out Nico. The reason of course is because Arthur was also left out. FN compresses time - having Barrie meet the Llewelyn Davies family in 1903 or so which was when Nico was in the midst of being born. FN could hardly have a pregnant Sylvia with no Arthur about!
I would definitely recommend that you read Nico's letters to Mary Hodgson - faithfully written year after year with his happy personality coming through very clearly. His handwriting is small, but very clear and he is a very good writer!
And here is a fan of the wonderful ghost writer M.R. James writing of his epistolary encounter with Nico (who was at Eton when James was provost)
http://www.users.globalnet.co.uk/~pardos/ArchiveMRJLetter.html