Go for it! I started playing D&D when I was 8 and haven't looked back. I agree with Chris that there is a difference between playing and GMing, but a 7 and 9 year old should be able to grasp the concepts of the game if they are interested.
I also agree with Herbo about tossing out rules if they don't work. I've run weekly games at my local library for the last year for teens and tweens. I never knew who is going to show up, so I developed a stripped down version of Pathfinder that is relatively easy to learn for newbies, but is faithful to the game.
I found that kids do better with role playing than remembering game mechanics. I would strongly suggest cutting out skills and substituting them with the appropriate ability rolls. This works well for most classes. You'll have to tweak the rogue, but it's not hard to give the class and a "Rogue Skills" or "Subterfuge" ability that allows them to do roguish things like pick locks and move silently.
You could also do away with skill checks and let them succeed or fail based on the merits of the story. One of the kids in my last game had a great suggestion and I let him succeed without a roll. He got really excited that he did something that wasn't in the rules and I think it hooked him on the idea of playing RPG's.
Good luck!