My mother and my father-in-law both ( at different times) had inoperable brain tumours. It is incredibly hard news to come to terms with and you have my deepest sympathy.
I think MB is right to pass on those words from her grief counsellor. Can you ask your GP about grief counselling and how it related to adult children?
I think you must do all you can to make it easy for your son and daughter to see their father as soon as, and as often as, they can. Can you offer to look looking after the baby, or if transport is a proble, offer to driving them to see their father or buy them train tickets. It is better if they can see their father sooner rather than later, especially if they want to say important things to him, or he has important things to say to them.
In my experience, the news can be more easy to cope with when you are face to face with the person who is ill, and can just 'be' with them. My mother sounded a lot worse on the phone than she did when I saw her at home surrounded by familiar things. In her case, her brain tumour affected her vocabularly quite quickly but she could communicate much better when I saw her face to face.
It was a horrible time, but there were some bright and very loving times that we shared- the tumour did not take away her personality and soul. I can only speak from my own experience of the illness, but I send you my thoughts.