Welcome to the forum. I'm sorry you've had to join us. I was diagnosed at 51 as well. Sucks at any age.
Well done on staying calm. Impressive! All reactions are 'normal' (whatever the heck that is!). I was beyond angry for several weeks. Put zero pressure on yourself to behave in any particular way. You will process what's happening to you at the right time for you. The only thing to be wary of is getting emotionally stuck at any point. Counselling can be useful at that point.
BCNA have this good page on telling your children about your diagnosis.
https://www.bcna.org.au/understanding-breast-cancer/talking-to-family-and-friends/telling-your-children/My experience was that it was important to deliver the news in an age appropriate way (my kids ranged in age from 18 down to 9 at that point). I told them once there was a bit of a plan, a date for surgery in my case. My situation was a bit different but I found there was a great exhalation when I resumed some standard bit of parenting or other, to go and wash their hands before dinner, or something like that. I can't recall it now. It's really important to show them that their lives, and your parenting of them, will continue as normally as possible.
They all reacted differently and I had to be mindful to modulate my behaviour to each child according to their needs throughout my treatment.
It's easy to disappear into your head while you're going through treatment. Necessary sometimes. But I soon realised that in the absence of information my youngest in particular was having a big worry and filling in gaps, incorrectly of course.
So if you do tell them this weekend, tell them early in the day, so you've had a chance to establish some normalcy before they go to bed.
Oh and tell them that people will bring lots of food and probably lots of yummy things for them! My kids quite enjoyed coming home and seeing what had been delivered that day!
Hang in there. This stage is the worst bit. Hard to believe now, but once your treatment plan is underway, you'll feel better. Big hug, K xox