I think both possibilities are right. (I was actually going to post a thread about this if there wasn't one already). I think part of that look could be explained by Peter missing his father. However, I tend to agree more with the other interpretation. Part of my opinion is influenced by Will's age, and therefore my vew on Peter's age (I feel Peter was about 16 in the movie). At least in America during WWII (and probably the case was the same or very similar in Britain) you were supposed to be 18 before you could join the military, of course, there are hundreds of cases where people lied about their age and joined at 15, 16, and 17. This boy is not much older than Peter--perhaps only a year or two, and he is going off to the front to protect his family and his country, while Peter is being wisked off to the country with the other "children." With their father gone Peter is the man of the house and as we see throughout the film and especially with his treatment of Edmund he is taking this role very seriously. I think being reminded that he is still a "child" really hurt him, and I suspect if things had been different he would have gladly been fighting along side that young soldier. Of course, it also makes for some great foreshadowing.