Here are some of my suggestions:
First - while you definitely need to tell them, don't feel rushed to do so.
We were in your shoes exactly 2 years ago and felt exactly the same way - we have to tell the kids but how can we possibly tell do so? We ended up waiting almost one month until MLK weekend (mid-January). The waiting was agonizing and I am not saying to wait that long (it made sense for us for a variety of personal and professional reasons based on the way we "rolled out our communication plan") but my point is, although you and your husband know and you have this huge weight on you, your kids and everyone else is blissfully unaware. So take the time...take it to figure things out and sort them through. Let your kids enjoy one more holiday season without this marking it.
Second - For what to say, I cannot stress enough - speak to Paula Rauch/PACT.
(or, if unable, read through relevant sections her book). You will get a ton of answers about how to approach this. My mind is a sieve and I forget specific details, but some of the main points you will want to make to your kids (and anyone else you tell) are:
We have difficult news to share...
- as you may have noticed, dad has been experiencing some (explain symptoms)
Give Diagnosis
(Also, especially when telling others, if the diagnosis is definitive, BE SURE TO state that (cannot tell you how many people challenge that, even medical personnel we have seen recently. "I know someone who had X, are you sure its not that?" "you know, lyme disease....". So nip it in the bud so you don't have to say YES WE ARE FREAKING SURE, THANK YOU)
Explain what this means
- while the diagnosis is certain, the prognosis /course of the disease is not and no two patients are the same
- Be honest - life-ending; no cure
Provide hope (but avoid false promises)
- will be getting best medical care
- it is a progressive disease so nothing is happening immediately, but should enjoy and appreciate our time together that much more
What can they do/How can they help
- for our kids - told them we are going to try to enjoy life together to the fullest and not take things for granted
- for friends with same-aged kids, give them guidance on what to tell their kids (you can be honest and say you are trying to figure it all out yourself) and how to interact with yours (we said treat them as they always have and provide love and support).
- if you are setting up some sort of support site (like lotsahelpinghands or meal train, include that information); consider designating one friend/family member to manage this to take off your plate.
We used the same basic outline for all our communications, just tailored it based on the audience. We actually came up a communication plan to agree on how to get the word out - who were the different audiences (kids, parents/sibs, extended family, close friends, extended friends, work colleagues, etc.); method (face-to-face, phone call, email, etc.) and timing for telling each group.