Tower,
I have three younger children along with older ones. My three youngest girls are 10, 12 and just turned 14. They were 9, 10, and 12 when I was diagnosed. I spent tons of time agonizing over what to tell them and found that talking to them openly then taking my cues from them was the best approach. I told them honestly that I had cancer,where it was and what the treatment for it would be. They asked if I was going to die and I told them not if I could help it, but that there was a chance that could happen. I let them know that it was NOT what the doctors, daddy and I were planning on but that IF that happened that they would be well taken care of. After that talk I answered any questions they had in the two weeks up to my surgery and did it casually and even with a little humor. After my surgery, my husband and older daughters brought the younger ones to see me even though I didn't look all that great, and they handled it better then not knowing if I was ok.
Give your kids the basic information then each of them will guide you as to how much they want to know. My youngest seems oblivious to it at times, but my 12 year old NEEDS to talk to me about it. Let them assist in your care, don't put on a brave face for them. My kids amazed me with how well they endured my treatment. Yours will also. The key to it, in my opinion, is NOT TO LET CANCER DEFINE THEIR LIVES. I was always on guard that they would become known as "the kids whose mom has cancer". The thought of that drove me crazy!!!
Take care,
Minnie