binky, my kids are about the same age, and my older boy 11, is the worst at doing exactly what you're talking about. Lately I've taken to interrupting him by saying, "Aden, I am talking. You must wait your turn." I've been doing it for about a week and a half now and it seems to be helping.
Personally, I don't think it's so much rude although it is annoying. When I say that I think it's an impulse thing in kids. Their brains are working so fast that they want to get every little thought out as soon as it pops into their head, so on some level, they're not even aware that the other person is talking because they've been busy thinking up stuff that they want to say.
I think it's a growing thing that most kids go through. With a little guidance I'm sure yours will realize what they're doing and start showing a bit more respect for your right to speak and be heard, just as I hope mine will also.
