Not everything is a dream, just the ending.
Cobb never found Saito in limbo, never woke up, and never went home to his kids. That was all a dream.
We know this because the kids are the same age, wearing the same clothes, and crouched in the exact same position they were in the last time he saw them.
But everything else in the movie, forming the team and performing Inception, was real. He just never woke up after he agreed to stay behind to find Saito.