As to the two sets of child actors, whose to say Cobb didn't simply them being older
in his dream? I don't think we can necessarily draw any firm conclusions just from that piece of info.
Upon the end of the film, I felt the character had completed his arc, and learned to let go of his emotions and fantasies of the past so that he could be mentally healthy and enjoy the reality of today. I never even thought that anything else happened, that feeling upon the ending is what felt natural and right to me. Once I started thinking that in the end Cobb was still dreaming, the film becomes weightless (pun not intended) to me, and merely an exercise in entertainment.
Well yeah, but an ending like this is kind of MEANT to hit you in the gut, and be a bit haunting and tragic.
To me, that's much more powerful and thought-provoking than if he had simply gone off and lived happily ever after with his kids in the real world. The simple fact that we want
that future for him means the movie has done it's job and made us care that much.
And in the end, maybe it's enough that he does get to at least see his children's faces again. He certainly seemed happy enough.