Hal, you know your kids better than us, but my wife and I decided that our son, who was 19 and away at university when I was diagnosed, should be the first to know. As Glen said, your kids need to hear it from you not from anyone else.
One of the things about this disease is that in almost all cases we and our families have time to come to terms (as best we can) with our lives. So don't take that time away from your kids and try to make good memories with them while you can.