I'm not a storyteller, but I have plenty of kids, and I do some teaching and coaching on the side...
20 minutes seems to me to be a bit optimistic on the attention span. The "experts" say plan on 3-5 minutes per year of age, but in a busy environment with distractions about, I think that would be high.
Definitely give them something physical to do, several times during the 20-minute period, to break it up into smaller bits for them.
Something else that works is to start off promising them something to look forward to at the end if they are good, like the answer to a riddle, or the punch line of a short funny story. That will help keep them on their best behavior, the little beasts.
Small children are great. Have fun!
-Glenn