I would also suggest visiting the school during lunch. I'd do this before getting the principal involved so you have a better understanding of the situation. Are you familiar with the child who is taking your child's food? Is it possible that this child doesn't understand that it's not ok to take food from your child? I know your child has talked to the lunchroom workers, and presumably they talked about this with the other child, but that doesn't mean that child understood. I work with Kindergarteners, and I could see several of my students doing this, given the opportunity. Not because they are trying to "bully" another student, but because the students I'm thinking of don't really understand social conventions like personal belongings/space (particularly students with ASD or developmental delays).
At any rate, I'd be sure that the situation really is originating from "ill will" before you throw the word "bullying" into the mix with school staff. It's a highly charged word and I'd use it sparingly.
