Thank you for all of the advice. I appreciated and reflected on every comment. I feel much more grounded after listening to your thoughts. It was good to be reminded just how straightforward kids are at this age. I think you are all right that Puppy will take this as it's explained to him for now. The basic facts probably won't trouble him yet, since the complexity behind the facts just won't occur to him. Any possible complications will come down the road. It always seems to come back to the basics: respect children's emotions, follow where they're pointing, answer in honesty and love.
I often do better in the moment with Puppy. It's easier for me to see things through kids' eyes--and put them into kids' words--when I'm interacting with them. Being surprised by this situation overwhelmed me and sent my mind into overdrive. I'm sure you're all shocked--shocked!--to hear I tend to overthink things. (Case in point: every other post on this blog.)
We won't talk to Puppy about it until we're closer to seeing her, but I'll let you know how it goes then. Do pray or wish good things or send positive energy for K's pregnancy.
And, Andy--I'm keeping your words close for later.
How did folks ever navigate unusual parenting situations like open adoption before the internet? Lordy, I'd feel so alone without it.