I've gone through the same thing over the past three years or so - my son is now 11 (and his sister, 3 years younger, sometimes jumps in). So, I'll just pass along my experiences.
A few years back, I was much more into the historical wargaming part of my hobby, and my son and I were playing "toy soldiers". I was dragging some books off the shelf, and hit on my stack of Monster Manuals (going way back to the old softcover Basic D&D versions and up to 3e), and my son did a "whoa - what's this!" sort of thing. Following a mental head-slap (why didn't I think of rpg's before?) I starting asking myself the same question - how to introduce it? We went to the hobby store and looked over the current D&D line, but to be honest his eyes sort of glazed over. It certainly seems to me that current D&D targets a bit older age group, perhaps 12-14 rather than 8-10 (especially if they have math phobia!).
At any rate, I started searching around, and came up with three different solutions. The first was the 1983 Mentzer Basic rules, which as others have said are simply the best example of introductory writing ever produced by the hobby. Forget the "old" style rules and such - the important thing is introducing the concept in a clear and fun way.
The second was Castles and Crusades, which for some reason has become some sort of controversial web topic, but in actual fact is simply an eminently useful mix of old and new D&D ideas. Although Mentzer was a great introduction, it was great to be able to buy something "new" for my son and his friends, and have them understand it all. On top of that, I could use my huge module collection, and all the old books that he was still looking through made sense in C&C terms, which wasn't always the case for current D&D.
I do think that Mentzer Basic - C&C - d20 is a viable way to introduce kids to d20, if that is what you want to do. To be honest, we haven't crossed the d20 line yet, as my son hasn't been terribly interested, and I'm waiting to see what 4e looks like (not too enthusiastic at the moment, but keeping an open mind). As an aside, the lack of a good introductory level game really is a weakness in current D&D, at least from a Dad's POV.
I mentioned three solutions. The third is actually not "D&D", but it made for some great gaming. It is the free rules set "Barbarians of Lemuria" by Simon Washbourne:
http://www.1km1kt.net/rpg/Barbarians_of_Lemuria_Free_RPG.php
I found it when pondering Sword & Sandals games (the old TSR Conan game, Sorcerer and Sword, etc.). It's a beautifully simple and flexible approach that is easy to explain, and easy to wrap around any idea that kids come up with for play, which do tend to be a bit more over-the-top in true S&S style. The actual text has some "colorful" illustrations which I would think twice about handing around to kids, but a little cutting and pasting and I made a short rules text to hand around. Again, it's not D&D, but it is an easy, flexible system to get them playing. You can certainly play in "D&D style", and we even did part of the Keep on the Borderlands with BoL - the PC's waded through goblins and orcs like chaff, which was fun, and the evil clerics in the Temple were suitably dark and impressive.
Again, these are just my experiences, and what worked for us. Best of Luck!