As 'pp123' says, you will probably be dismayed to find how quickly a half hour will go!
I think in a task such as this, it is not so much the 'educational' value of the lesson that is important, as the way you interact with, and control the group, keeping the children entertained and happy!
I didn't know the Granny story, but just looked it up. You might need to adapt it, to keep it as short as possible, and limit the number of places the carpet flies to, and the objects collected.
Young children sometimes don't have much concept of 'other countries'; anything outside their own town being considered foreign.
Using 'props' and objects is a great way to keep children engaged, so, Yes, sitting on the carpet should stimulate their ideas.
Watch out for the 'chatty madam' who wants to monopolize your lesson, and try to subdue her enthusiasm. Conversely, if there are shy, quiet ones try to coax something from them, but don't pressure them so that they dissolve in floods of tears!
[Thirty years ago, when I first helped in a primary school to coach children playing percussion for a Christmas production, I put a bit of pressure on a girl I KNEW could do what I was asking, but she wasn't producing. She cried, and of course, I felt awful. The rest of the group said: "Don't worry, she ALWAYS does that!]
[When our own child started school, I became a 'parent helper', supporting reading one morning a week. One of my first 'readers' turned up again seventeen years later, when she was in her final year of Teacher Training. Our roles were reversed, and SHE needed to direct ME in the classroom.]
I don't know if you can 'learn' the story, so you can tell it without reading, and thus keep an eye on the children. Have the book there, to show them pictures, and try to make sure they can all see them.
You may not have time for this, but I often suggest making a 'deliberate mistake'. Children love thinking they have caught an adult out, and you can act mock embarrassment.
A brilliant head teacher I once worked under, unexpectedly had to take a Reception class after their lunch break. The children were very restless, and without saying a word, she calmly sat there, and slowly tore up pieces of paper, dropping them on the floor, until she had all the children's attention and complete silence!
I assume you have worked with this age range before.
Try and relax, and ENJOY it; chances are, if you are enjoying it, the children will be too.
Come back it you need any clarification on anything.