Baggy, that's really sad news, I'm so sorry and I can totally empathise with you. We lost my FIL to a brain tumour on Christmas Eve, 3 years ago. My son at the time was coming up to 5. He knew that Grandad had been ill (diagnosed in the August and got progressively worse) but telling him his beloved grandfather had gone to heaven was one of the most difficult things I have ever done. Having said that, I'm glad I did it straight away. He was a bit sad, mainly for my husband (he's a sensitive soul). Unfortunately he lost one of his friends from having been a baby to exactly the same thing earlier in the year, so we'd already had to explain in quite simple terms about heaven.
Bizarrely, Christmas day that year was actually quite "normal" for a couple of reasons: it hadn't quite sunk in yet and we knew he would not want Christmas for our son to be ruined. So it gave us a focus, which helped us all, particularly my MIL.
As others have said, you know your children better than anyone and whatever you do will be the right thing. They are a lot more intuitive than we give them credit for and sometimes, young children in particular deal better with these kinds of situations as long as you give them some simple information because unlike adults, emotions are not as developed so they process things more logically.
Good luck with your journey, it will be tough but from what I've seen, you're strong and will get through it admirably.
x