Never underestimate worms and a bobber. Get the 6 year old a character pole (princess, Minnie mouse, snoopy etc) and the 12 year old a zebco 33 combo. The one with a little tackle included. Go to any local creek, and bait their hooks the first time explaining that they will do it next time. Let them wade the shallows and throw some rocks.
And as was mentioned, when they are done, move on. That's my hardest lesson, but I have a 19, 12, and 9 year old. They each like to fish but not anywhere near as long as I want to.