I, too, had a lot of questions about who was dreaming and whose subconscious was controlling what. For example, in the last level, the limbo level i suppose it was, why are they in a city that Cobb built with his wife? Or I should say, why would the rich guy and Ken Watanabe we included in that? The only thing that would make sense, to connect all the worlds, was that it was always Cobb's dream.
As far as his kids not aging, i dont think we know how much time has actually passed in the presumed real world. Yes, he spent an eternity in limbo with his wife (till the grew old apparently), but that might be only hours in the real world.