Pack preschoolers' lunches in waxed paper bags
My daughter goes to the most wonderful preschool in the world. It's the same preschool my son went to, Waldorf-inspired, totally magical, very simple. The teachers there often send home little things the kids have baked or prepared in unbleached waxed paper bags, and I couldn't get over how much nicer they look, feel and sound than plastic sandwich bags. (Strange that I had such an aesthetic response to SANDWICH BAGS, but there it is.) Better for the environment, too.
I bought a box at our local grocery store, but only after packing a few of my daughter's lunches did it dawn on me how much easier it is for her to open these bags than the zipped-shut Ziplocs or reusable plastic containers I used to use. For preschoolers just getting a handle on manual dexterity (nice developmental pun there, hey!), any chance for them to do something independently is good -- and opening one's own lunch falls into that category.
If you can't find bags, wrapping items in regular waxed paper works well, too.