Hi Clive...
It looks like you've been given a lot of wonderful advice and ideas to help you get started, but I did want to jump in on this one as well because I specialise in fun learning games for this age group.
I do have to say that I agree that 20 minutes is often too long for this age group as you may not be able to hold their attention that long (sometimes they'll surprise you, however). The idea of setting up a general schedule for them is also a great one because kids of this age like to be able to predict what will be happening next.
Since kids of this age don't know how to read or write much more than their own names, playing games is the way to go. When teaching with games, especially to young children, you'll want to take into consideration the competition factor, however. While competition is often a motivator for older kids, preschoolers can easily get discouraged if competition is weighed too heavily on the game, so you might want to consider finding ways to make everyone "winners" or just play the games "for fun".
I hope this helps!
See my
preschool games book for tons of short activities for kindergarten kids.