We have a 6 year old and my husband was just diagnosed with colon cancer, so we're in the same boat. His hair won't fall out, but he needs 24 hour continuous chemo followed by some heavy duty surgery, so it's not something that we'll be able to keep a secret.
I checked with our pediatrician, and she said to be honest, but give as little info as possible. If our daughter asks for more information, give it in small chunks. We'll probably tell her that daddy has something in his body that could make him sick, and the doctors are going to give him special medicine to make it small, and then he'll have surgery so that it comes out for good. The medicine might make him feel bad, like the way a shot hurts, but just like shots it's to keep him healthy in the long run. Our doc recommended that we stay away from the word "cancer" until she asks. I'm not sure that we're going to go that route; a big part of me is inclined to say that the thing in his body has a name, it's called cancer, and it's something that only happens to grown ups. I just don't want her to overhear the word and think that it's something so scary we can't talk to her about it. But we're not quite there yet.
I wish you the best of luck. I am so sorry that this happened. It's scary as all hell. If you want to PM me, you're welcome too. I know that it's not me that has cancer, but it was pretty clear pretty quickly that this is something that happens to a whole family. I'll be thinking of you.