I do this with my kids for written narrations, and only from about age 10 and up or a little older. It helps them learn to take notes and organize their thoughts to write.
But for younger children, I wouldn’t bother. 20-30 minutes of reading is too long for them to remember it all if you are wanting them to include everything they read in their narration. I would rather ask something like, “What was your favourite part?” or something a bit generic, rather than requiring them to tell you everything that happened. If you want them to re-tell the whole of what they read, I would make the reading much shorter.
I hope that helps and makes some sense.