>>>We know they don't act it out by actually moving around and doing the actions, apart from the very end, because Patrick says so, but are we to imagine them taking it in turns to narrate a kind of recitative, something like the italicised narrative the reader gets? Or just speaking their own parts?

I don't remember speculating about that. At that time I was much more familiar with the C S Lewis type of a fantasy world - all on paper, except when the toys were used to act out the scenes.
We all had "puppet theatres" during the 1950s, and the puppets were usually home made. I think that AF was ahead of her time in seeing the potential dangers of role playing games.