"Certainly they were children again -- but why would they lose their nobility of spirit?"
inky, if they lost their nobility of spirit, then Edmund would have returned to England as the exact same boy as when he left, and that wasn't the case. Eustace as well.
Why would anyone return and become worse then when they first went in? That was not the point of Narnia. It was a place where one learned lessons and kept the knowledge of those lessons.
"Edmund and Peter do that in the book as well, the difference being that the fight starts just as they intervene with the werewolf leaping on Caspian."
Asbel, the other difference is that in the movie, Peter and his siblings have to come to the rescue while in the book, Caspian and his allies are already fighting, turning Peter and Edmund (and Trumpkin) into more or less back-up.
MrBob