Another dedicated LL Bean user here.
Order NOW. In August, they will be backordered for the school year. Even if you order two now and let them decide which they like, you can always return it. Better that than starting the year without one.
Even the LL Bean Jr. backpacks will hold a full-sized folder. For K and 1st graders, I would not recommend the larger ones -- they are too big for these little guys and gals to carry comfortably. For this reason, I would not buy a large one now and have them use it 'until they grow into it'. If you get a great deal, I would keep it until they are large enough to carry it. If your child has to once a week carry something outside the backpack, that is better than creating back problems now.
No matter how good a lunchbox is, it stills get crusty after 180 days of spilled milk, OJ, watermelon juice, etc. Having said that, I still buy LLBean b/c of the quality. I know it will last all year! I don't like the 'foldover top' ones -- I can't fit things in w/o them getting crushed. I like the 'lay flat, zipper around' ones instead.