To get away from using plastic containers from your child's lunch -- to pack a completely eco-friendly lunch for them. You want to start with the lunch bag itself. It can be cloth and have absolutely no plastic in it. What you put inside there, you can put a cloth napkin instead of paper napkins. You can also replace all of your plastic containers, all of your ziplock containers, with stainless steel containers.
You buy these once. You will buy a nice square one for sandwiches. You buy a nice thermos, in case you want to send some soup or something warm. You buy a couple of smalls ones for fruit and cookies, that sort of thing. These things will last your for years. We are working on the fourth year that I bought for my child when she was in Kindergarten. She's using the same set, four years later.
Another great option are cloth ziplock bags. You get rid of the zippies. You get rid of the plastic zipper bags. You teach your children that they can help the planet and that they are helping their bodies at the same time, by keeping away from the plastic which can leach into their bodies.