For me there are two separate issues. One is just taking them along hunting and the second is actually allowing them to shoot and kill an animal.
As far as the first, I don't see a problem taking kids along atabout anyage. As long as they are warm and comfortable and aren't made to stay out all day, go for it. In fact weren't ground blinds invented just to take kids out hunting? LOL!
Plus walking around the woods just looking for sign is a great thing to do with kids.
The second is a little more tricky for me personally. It's been pretty well studied that kids don't really fully understand the concept of life and death until around the age of 9. Again this is a personal issue with me, but I have a problem with putting a firearm in the hands of a younger child and asking them to take a life before they understand what that means. I won't go into here, but I've had a personal experience thay has really shaped my opinion of this.