Originally Posted by Max Demian:
“How is it actually done? I don't recall any "making of" features.”
As I have heard it described, the adults have the outline of the plot and a number of scripted lines. They know the beginning, middle and end of the episode and the general path it's meant to take to get there.
The children have a general idea of what's it's about and hints of what sort of things they should talk about, but no (or certainly few) actual lines. For example Ben might have been told to talk about "things Jesus could do" but he is then left to come up with teh actual questions (e.g. "What if Jesus was attacked by a polar bear").
The adults therefore have to improvise to react to whatever the children say, run with that a while, before eventually getting the episode back on track heading towards the next improvised bit.
(And then there's lots of editing)