Oh Crisps I'm really sorry to read this.
My two were 13 & 10 when my DH (not their dad but we'd all lived together for while) was diagnosed last year.
Initially we didn't say anything - it had all been very quick from 1st symptoms to initial diagnosis of the primary tumour, he then had further scans and tests and we were told it was inoperable with a 12 month prognosis.
At that stage I told both boys, separately, that he had cancer and that it was inoperable. My 10yo accepted that and my 13yo questioned what it meant - so I explained further based on the information we had.
We continued to talk about how he was, his illness and his treatment but ds2 never really questioned what the ultimate outcome would be.
DH had chemo and responded well and had an improved 2-4 year prognosis last September, but then went downhill and after lung mets were found in January they said a 3-6 month timescale. At this stage I discussed the situation further with the boys (obviously they'd seen him going through the chemo, and then radiotherapy, and basically becoming more unwell).
DH died in March and for the most part they are taking things in their stride.
I fell apart last summer, and had some sessions with the pychologist at Maggie's, I'm not sure how much it helped - I think the main thing was time in coming to terms with the situation.
It's such a massive thing, and it does consume your life. Try to be kind to yourself, you need your strength as well.
[Willowkins I'm glad he's doing well].