Clearly the ending is MEANT to be ambiguous. Anyone who argues that it's NOT meant to offer at least 2 possible interpretations is being disingenuous. Yes, it is objectively strange that the kids are in EXACTLY the same position at the end as they are in his last memory of them. Is it impossible? No, just strange. They're also, I believe, wearing the same clothes. Again, not absolutely impossible, but very strange. Also, their being the same age - strange, and especially strange considering the movie DELIBERATELY avoids mentioning how long Cobb has been in exile. It would have been an obvious thing to put in the script somewhere, and he deliberately leaves it out, so that it's, again, possible that it's real, and still very, very strange.
And the spinning top does, objectively, begin to topple for about a millisecond, and then more or less straightens out. And again, this is done to leave both options open. Of COURSE the top doesn't absolutely fall, or absolutely spin forever - the only reason it's effective as a last shot is because it's ambiguous.
It's also brilliant. Reminds me of the end of Philip K. Dick's Ubik, where the guy finds the face of his employee on a coin in his pocket. Anyone who's read the book knows what I'm talking about, and knows that this ending may very well have been inspired by it.