Hmm...I've never read Peter Pan in Scarlett or any of the other attempted sequels, so I don't know what's inside them. Have you? I saw Hook as a child, and while I appreciated it for Spielberg actually reading the book (more than could be said of some other adaptations), I found it to be rubbish too.
Funny story: I'm guilty of writing my own fan fiction sequel as a 15 or 16 year old, called "Kiss of the Lord" but it was awful, and only one other person has laid eyes on it (note to self: go in the attic and burn that as soon as possible). I remember trying to incorporate as much as Barrie's original as I could, but then added so many of my own characters and plots from my personal life that it made no sense as a sequel. It was more of a coping mechanism for an oddball teen, and it ended up changing my life, so no regrets. Sorry, got sidetracked there.
I'm all for fan fiction, professional or just for fun, and sometimes will go as far as death of the author, but I've always made the exception with Barrie, because of his life story being so intertwined with his work. So I wouldn't be easily impressed with any attempted sequel.
As for Wendy's children and grandchildren, I believe Disney did a movie theatre sequel about Wendy's daughter Jane. Apparently, she is in the middle of the Blitz and having trouble in growing up too fast, and ends up in Neverland where she learns to believe. Something like that. I never heard a story of Margaret though. I'm currently reading a novel about a great-great-whatever grandchild of Wendy, named Fergus, and he has Autism (going to Neverland helps him learn to cope with the world and find confidence in who he is). It's not very Barrie-ish in style, but I have a spot in my heart for people on the spectrum as I work with children who are.