I think this is stupid on so many different levels. First, the whole idea of choosing someone to write an official sequel to Peter Pan is patently absurd. Let whoever wants to write write, and readers will decide if any of them are worth reading. Barrie's story may be copyrighted, but that can't apply to other works inspired by Peter Pan. Hell, just about every work of art is inspired by or is derivative of something else. You don't see the Baum estate suing Gregory Maguire for writing an Oz prequel, for instance. Should Gardner have been prevented from writing Grendel? (Okay, I know this is an absurd question, since Beowulf isn't copyrighted.)
Of course, I always found the entire idea of copyrights in arts and letters detrimental. Rather then allowing ideas to germinate, it stifles them. Artists should be encouraged to share ideas and borrow liberally from each other. They shouldn't have to worry about pecuniary repercussions from doing so.
And, an official sequel? Hell, just the mere idea of a sequel seems ridiculous, official or not. It seems like a transparent ploy to make money, to capitalize on Barrie's legacy. The chances that McCaughrean will capture the same magic or the original Peter Pan seems unlikely. <sigh>