6 1/2 is perfect! Get 'em out there young! Like VA said, she'll get restless. Your hunt WILL be shorter than you'd like. Take some coloring books, gameboy, psp, snacks, drinks, earmuffs are good, since most kids don't like stuff in their ears. Don't forget the toilet paper and show her where she can go pee. If she doesn't already know how to pee in the woods (it's different for girls than it is for guys) here's a little tip: Find a sapling that she can easily grab hold of. She needs to face the sapling, grab hold and sit back, then she can do her business. Simply squatting to pee will result in wet pant legs and boots.
Shoot, let her work the call, too! The kids and I have fun trying to sound like a bunch of turkeys.