One thing that makes me feel that Cobb dreamed the end is that the faces of his children were never revealed.
And weirdly, they don't seem to have grown up either. Children of that age change significantly in a few years.
Somehow I feel Cobb sees only things he can remember. He sees them how he has seen them last.
The 'not showing their faces' thing makes me feel that maybe he didn't actually have children. Maybe Mal was right when she said that they weren't their children.