That's cool. I want to get my kids into it too. I think they'll be ready in a couple years. When I first started playing back in the mid-70s my group included two 9 year olds and a 7 year old. The 7 year old was a little on the young side, as I recall, but the 9s did a great job.
Yep, 4th edition. Having tried each, I think I like this version more than any previous version except the first. They've changed quite a bit to make things easy, but in so doing they haven't dumbed it down, just made the way the system works more logical. Previous versions were rules heavy. This version seems light, but that is because more things make sense.
Rules are good. Fewer exceptions to those rules is very good.
Son has been getting into the Warhammer 40000. Has a bunch o' miniatures, likes the Blood Angels he does. I think it has to do with the word "blood". I do think that maybe in about a year or two he should be ready for D+D. He could prove me wrong though.