Dang it. They could film The Magician's Nephew any old time since it would not require any of the children from the previous films, but if they film it now and try to film The Silver Chair later, they will probably have to recast Eustace, thus making the films feel less connected to each other. For that reason I wish they were planning to film Silver Chair next.
Having said that, if they are indeed hoping to film all seven books, perhaps they want The Magician's Nephew filmed before The Last Battle, and they want Last Battle filmed right after Silver Chair so that actors playing Eustace and Jill won't age too much between those two films. Could that be why they want to film The Magician's Nephew next?
If so, since Magician's Nephew and Silver Chair would have few actors in common--just Liam Neeson and possibly Tilda Swinton--I wonder whether Walden Media could afford to film them at about the same time with one director working on one film and a different guy directing the other. That way they could keep the kid playing Eustace. Somehow, I don't think that's likely.
That could also help prevent too much time from passing in between the releases of those two films. That is part of what I believe hurt the opening weekend of Dawn Treader; enough time had past for people to forget the series and lose interest.