I'm a Lunchtime Assistant [waves cheerfully at Sunny and DarkEvilMoon] I don't police lunch boxes, thankfully, because though the HT released a lunch box policy a few years ago he has never done a damn thing about enforcing it. I do know, because I have eyes and the boxes are open in front of me, that the contents vary in the standard view of nutritional value. Even as an ignorant old pussy 
I've managed to figure out that a chocolate spread, on white bread sandwich, with crisps and a chocolate roll from a multi pack is generally down the scale a bit from a chicken on homemade, whole wheat roll with carrot and cucumber sticks. I also know, from some of the parents, that some of the more startling lunches are down to dietary or sensory issues, so I've learned to mind my own business and also defend children when those around them decide that they should be the lunch box police.
Our school lunches are made with meat from the butcher a mile up the road. Today's lunch was actual, unprocessed chicken breast portions, new potatoes, peas and sweetcorn followed by an oat and golden syrup cookie (I know, I know).
And (not because it's relevant to the OP, but because it's come up further down) I have no truck with "Boys will be boys". Any sort of aggression or meanness is dealt with calmly and firmly and promptly passed to their Teacher at the end of lunch or the HT immediately (depending on the severity) whatever the gender of the child.
It's been said before, but, if my experience is anything to go by, most Lunchtime Assistants are just plonked in the playground with no training other than what they get from the existing ones (I got to shadow my predecessor for one day and then I was left to my own devices) and little support (It was only at the end of my first year on the job that we got the HT and Teachers on board with any sort of organised playground behaviour management), so if you are having issues then you need to keep on at the school, it can be done and it can be done well, or at least, better.