I do not understand expressing concern over anyone else choice of ONE lunch when you have zero idea if that ONE lunch reflects a pattern. It just seems very judgemental and that is why the Op is getting that sort of response.
How can the choices of a packed lunch for a field tip day be of any concern to anyone? Why assume the parenting role for someone elses child, by being 'concerned' about their health because of one brown bag lunch?
Unless their lunch bag had weapons, illegal drugs or alcohol in it, I wouldn't give it another thought. None of my business.
Also, how would you react if someone came her and said, "you know...I saw this kid who opened his lunch on field trip day and the mother didn't give him any treats at all... I mean, what kind of mean mommmy would not try to make the day special with a special lunch? This poor kid had to sit there all sad and see all of the other kids with the special lunches their mother packed and this poor kid had just ordinary every day food. His mother obviously doesn't care that this is a special day for him"
IMO, it's the same thing. Judging the parents based on one small lunch and no other information, out of 'concern'