Unfortunately, gargoyle, Alfonso Cuaron ("the same Mexican director") is not doing number four, which is being filmed already. I believe the new director is Mike Newell (Mona Lisa Smile, Donnie Brasco, Four Weddings and a Funeral, etc). His main fantasy before this was "Into the West" which I saw but don't remember being blown away by; it certainly doesn't stick out in my mind as either awful or brilliant!

To deal with the problem of the kids aging, they could always go the route of filming several books at once, as they did with Lord of the Rings. The only problem with that is, 4 and 5 would be the obvious choices and as far as I know they are not doing so. (They could also do 5 and 6 together; book 6 will probably come out just in time for them to adapt it and get rolling!)   

I also want to emend the comments I made before re: race. Several people already pointed out that, as in Tolkien, different species stand in for different races, too. There is definitely a growing unease among the children--and the adults too--about the way elves, goblins, and Giants have been treated. I think a social transformation is beginning in which some of these "creatures" will have to lend their help to the forces of good in order to destroy Voldemort. So not only is there the question of "muggles" and "muggle-born wizards," which is perhaps vaguely similar to anti-semitism (they are somehow foreign, yet seem perfectly assimilated, like England's Jews), but there is also the question of more obviously, physically different "races" that have been oppressed.

(I must say, the Goblins also remind me of Jewish stereotypes common in England from the middle ages to Victorian times and beyond; they are small, with big noses and beady eyes, they are shrewd and clever but suspicious and unfriendly, and they're all bankers!)