My boys are a little like this too. I think boys are definitely more prone to this than girls. But we have a very open atmosphere in the house, and they see us naked. They just have to learn what is OK and what is not, and that takes time when you are this age.
My 2 were worst for flashing their bottoms around, and we have had to go down a firm route with that one!
But in general little kids have no real inhibitions... perhaps if my body was in such good shape I´d want to run round naked too.!