Before you get too concerned, we're all fine. Here's what happened:
F's highchair tipped over sideways as we were getting ready for lunch. With her in it. SP was the source of movement, but it wasn't intentional.
So thankful for our little wooden house with its lino-covered wooden floor. Tiles on concrete would have been so much worse.
As it was, she was crying immediately, then going a bit floppy a minute later, so I called the ambulance. SP was so good, trying to be helpful, watching for the ambulance. So was our postie, who happened to be passing. A few scary minutes waiting for the sirens, but once she saw the unfamiliar people she brightened up and cried nicely :)
A few hours wait in emergency and short stay units for observations, and we were home for a slightly late bath time. Both kids now in bed asleep. SP will have lots of questions in the next few days I suspect. But he'll also know how to dial 000. And it's no worse than happened to his dad and aunty thirty years ago...