Since there have been a bunch of other things I consider spoilers upthread I won't sblock this.
I'm on the fence about whether the ending means he's still dreaming or not. The children not aging really does it for me (when he spoke with them on the phone they sounded older than that, IMO), but I actually wanted the ending to be ambiguous - I didn't want to be able to come to a conclusion about it. I like it when a film keeps me guessing at the end.
.
I did not notice this the first time around in the movie, but if you know what to look for within Inception, it is very clear whether the ending is a dream or reality. I had to have it explained to me, and then see it again. Then, you say, "Oh yeah, now I know."
It is NOT related to the children and their clothing, either.