This forum has been similarly invaluable for me @wren71. I'm sorry the news was not good, but that's one ghastly thing over and done with. It's good you're seeing your BS this week, he/she will have a plan for you and you'll go into the weekend with a bit more certainty. You'll be able to convey that to the kids.
Have you got a breast care nurse yet? And the My Journey kit? Both can be amazing. Lots of advice on how to handle the kids.
My advice would be open and honest with them. Kids fill in gaps if they're left blank, and sometimes erroneously. I have four (with particular circumstances) and found that two were good with whatever I told them, but the other two needed more detail for reassurance. Sometimes I disappeared into my own head and forgot to communicate which led to consternation, worry and misunderstanding.
So, more breathing! Have you tried 4-7-8? It's very good for calming the nervous system. You breathe in through the nose for 4 seconds, hold it for 7 seconds and then breathe out through your mouth for eight seconds. Do that five times. I find that very helpful, including when I'm trying to get to sleep.
Big hug, K xox