I think it depends on the kid, I have one semi-hyper personality and one laid back personality in my two boys. I was able to put the easy going one in a treestand with no sweat when he was 8 and could have done so sooner except that I was worried about him physically being big enough to climb the sticks, the hyper one was 9, but BOTH were belted in something fierce. I could not bear the thought of one of those guys tumbling out of a stand and it being my responsibility. If that wouldn't be bad enough I don't think I would live through the "opportunity" of explaining to my wife what happened. Safety first!
Keep it fun, don't make them "hunt hard" at this stage. There just may be a few giggles, snorts and guffaws at inopportune moments and you will have to expect that. Take a camera and take some pictures of them there in the stand with you too. Let them pick up sticks, acorns, feathers, etc, they think that is pretty cool at that age. I let them name the deer they see and they come up with some dandy names too.