My son's BOB usually includes an LED glowstick, a favorite stuffie, a couple hot wheels, a couple of little green army guys, a deck of cards, a book, a racecar blankie, bright orange hat, gloves, fleece jammies, extra unders and socks, little toothbrushing kit. It's also usually got an mp3 player, headphones and spare batteries.

He's a smidge older (8), but he carries one water bottle, 1 juice box and some of his favorite snacks. He's an overpacker like me so I watch that the bag doesn't get too heavy. Tupperwear might work better for your daughter. The trick is finding something that she can open and close, and not making the pack too heavy. Ziplock bags?