I would start by going in at lunch, introducing myself to whoever is watching over the students, and eating lunch with my child - and seeing who's in charge and to meet the child who's stealing your ds' lunch. It's possible (depending on the child involved) that something as simple as you sitting down at lunch for just one day, and telling the other child that you know he's been taking your ds' lunch and you are going to report it if he ever does it again, might be enough to stop it. If not, you'll have made a contact with the person who is in charge during lunch, and that would be my next step - let him/her know what's going on and ask for them to handle it however you think is best (separate the kids, watch out to catch the student in the act, whatever). Then after those things have failed, I'd send an email to principal etc as suggested above.
The reason I'd start with the lunchroom visit etc is - I saw a lot of things like this go on in my children's early elementary lunchroom on the days I happened to be there at lunch time, but the teachers aren't typically in the lunchroom and the adults who were charged with overseeing the kids had a huge massive bunch of chaos and noise to deal with, so it was easy for students who "quietly" picked on others to slide under the radar. When you go in, all of your focus will be on this specific situation, and that in and of itself might be all it takes to get the other student to stop.
Best wishes,
polarbear
