Ann Althouse posts on the fact that in other societies young children work and that it might be good for them. She focuses on the long hours of sitting and mentions a new study that finds sitting almost as bad for your health as smoking.
I don't know about that, but it did put me in mind of something that I heard from a friend just the other day. She said that since her six year old daughter has had to take care of my friend's bed ridden Mother she has become very responsible. It has apparently been a huge change.
I think it is not so much the physical activity (though having 7 year olds sit for 7 hours a day is surely neither good for them or very natural) as the responsibility that has the effect. There is something that matters, something that will go wrong if they don't do their jobs. It is an unnatural state for human beings to not matter. For most of its existence the human race's living standard was so low and its means of sustain itself so precarious as to make it obvious to all how their contribution mattered. We have gained something, but we have also lost something.