Hi, my 7 year old son is not a vegetarian, but he is very particular about what he eats. I have been fixing his lunch since he was in pre-k. He has not grown tired of whole wheat bread with peanut butter & a little bit of jelly. For two years, he only had the peanut butter in his sandwich; we just started adding a little jelly this year (ever since he started staying over grandma's house).
Along with the sandwich, I keep it simple but will mix up the "sides": fruit roll-ups, various types of gold fish, pretzels, plain chips, etc. with fruit juice or lightly flavored water. It's been easy to do -- five days a week, and so far, it's just enough for him to be content at school. I'll add a little something extra for a snack. Now, of course, as he gets older, I'll need to add in more, but it is like pulling teeth to get him to eat "fruits and veggies." I can't fight that battle with him while he's at school, so I fix what I know he'll eat. The most I see him moving to now is...an extra sandwich. :-) Hope this helps.