Now that you have read this far, I don't have all the answers not now, or maybe never. What works for you may not work for others so it is your path that you need to find. Regardless of what is happening, I will say that you must talk and know that even if you don't think you should talk to them, do so anyhow, because they are already aware that something is going on. Kids are much smarter than we give them credit for. Now of course, it goes without saying that what you say and how you present it depends on your child and their development and what they will be able to understand.
I do know that throughout your conversation, let them know that you love them and hugs and kisses will always be willingly accepted and given as they can't catch what you have. Also let them know that this cancer is not their fault, they did not give it to you. Like adults as soon as the word cancer comes out, our mind kicks into overdrive and we think of all the things we have ever seen or heard or associated to it. Kids will do much the same and television has helped to provide them pictures and images but not necessarily correct or appropriate ones for what will be happening next in their lives. Initially, there may not be many questions or comments but allow them time to process this information and in the meantime try to maintain your homelife as much as you can as before the diagnosis. Laughter is one of the best tools you have and let them know, it is ok to laugh and to have fun and look at ways to celebrate your family and get them in on the act. If you are going to loose your hair, how about letting them shave your head when it becomes necessary or have a hat party and everyone finds or makes a hat for you.
Despite all your efforts, I guarantee that information will filter to them through outside resources and there is no way to stop that but guide them as it happens and listen and allow them to talk. It may take several days to get to where you want to go with your discussion but you are equipping them with the tools they will need as you progress through your treatment. I don't recommend young kids come to the medical facility as those sweet little ones can be carry some little germs with them that could be costly (healthwise) to other patients as they may have a very low immunity and easily can pick up other illnesses and that can also delay their treatment or add to their burden. Older kids may want to just come and see what it is about and given that, see if someone can bring them in while you are going through your appointment but also make sure they have an easy out if they are ready to go home before you are done. Again, I will impress upon you that it is so important to talk to them and to listen. It takes far too much time and effort to try and hide what is really going on. Keep communication open throughout your treatment and look for ways to celebrate life.
Years ago when my children were home and small, there had been alot of tension in the house ( now granted I wasn't dealing with cancer at the time) and I looked for a way to break that tension. I bought a can of whipped cream for each family member and we went outside and had a whipped cream battle until we all just melted in laughter. It was an inexpensive, fun way of letting go but yet reconnecting.(HINT: wash clothes immediately after you are done playing or they will smell like spoiled milk if you delay-LESSON LEARNED).
Having said all that and hoping you give yourself time to understand your diagnosis and what is next and then planning ahead and putting your thoughts together to tell you kids and grandkids and preparing them to