Hardcore Squats and Deadlifts with regular beatings to improve morale.
Definitely something overhead. With my little nephews the first thing they do when they see my 1kg plates or dumbbell handles is put them overhead. There's something quite primeval about that.
I've got my little nephew (8 years old) doing Squats before. The only thing that is a concern with these little guys is that they have absolutely no fear. So he just crashes into the bottom position with no regard for his knees. I have to constantly slow him down.
I'm hoping to train my son or daughter whenever they come along as early as 10-12 and I'm hoping to just teach them technique in a wide variety of Power and Olympic lifts.