My son (6 yrs old) loves cherry tomatoes. They pack well with lunches. Try half baby carrots and half tomatoes mixed. He also loves cucumbers, but they can sometimes get a bit slimy in a plastic bag in a lunch box. We're just about out of growing season here, but are you in a climate where they can grow their own fruits/veggies - that always makes them taste better to kids.
You can put almost anything in a whole wheat pita shell or tortilla. You can do savory with lean meat and veggies, or go sweet w/ fruit and honey or fruit and a bit of yogurt and fix it as a roll up.
Try making a chicken salad with diced up chicken breast, quartered grapes, a little plain yogurt and some sweet/spicy seasonings (cinnamon, ginger, cardamon) or cumin/salt/pepper. Can also toss in some chopped cashews. Sounds wierd but tasty. The kids should like the sweet and they wont realize its good for them.
A cut up banana drizzled with a TBS or so of melted peanut butter and a sprinkle of cinnamon also makes a good snack.
Also just plain old lowfat string cheese. Yummy.
Plain popcorn sprinkled w/ a bit of sea salt, pepper, and parmesean cheese. Much less fat then the butter drenched kind and still tasty. Or sprinkle the popcorn w/ the previously mentioned sweet spices. Yummers!!!!
Ok, I'm hungry now, but I hope this gives you some ideas to get started
Nan