....I wasn't faced with the long illness of my partner that you have to deal with, but have some relevant experience that may aid you.
....Mrs Goth collapsed and had a cardiac arrest suddenly 2 years ago, was resuscitated after an hour and was ventilated on ITU. The urchins were in bed at the time, so received the news in the morning. Resuscitation is part of my work, so I knew that long term survival chances were nil.
...the urchins at that time were 9, 7, 5 and 5. The 9 year old asked me directly if Mummy would be alright, and I answered truthfully "no". He was upset, but at least he knew what to expect.
...after 3 days in ITU all tests were complete. I took them to school in the morning, having told them that Mummy was going to die later, and we would go and kiss her goodbye. The younger ones (more relevant to you, as similar age to yours) were surprisingly matter of fact. They did cry, of course ("who will be our Mummy now?") but you will be surprised by the questions.
....they wanted to know lots of factual information. Questions which from an adult or older child would sound ghoulish, but they simply just wanted to know. Apparently this is commonplace in the agegroup. I answered these questions in the matter of fact "quest for knowledge" tone in which they were asked. It was hard, but I think the best thing to do. They are now happy, healthy 7 year olds.
The older two were slightly more problematic (my now 11 year old is being assessed as likely Asperger's, and the now 9 year old has always been, er, spirited) but I am convinced that it is best to tell them the truth. Help is available. Winston's Wish are a fine resource, and I found that they give sensible telephone advice. Unless you live in Gloucestershire that is all they offer, but nonetheless I found it useful (and I am a cynical soul, reluctant to accept the platitudes of yoghurt weaving yurt dwellers). I am sure they would give you aid that would be of use now, as will a hospice (if you are involved with this service).
....I thought at the time that if I could face telling this news to the urchins then nothing would ever faze me again. Alas this is not so (I have already been exasperated this morning by arguments over seemingly identical bikes. Don't ask) I am sure, however, that direct truth telling and earnest answers to earnest questioning was the best approach for us.
Good Luck.