I know this thread already abounds with great advice and links, but I wanted to share my experience as well.
I used to teach this age group (Korean 5 year olds, which range in western age from just under 4 to nearly 6). I saw two groups for 80 minutes each 5 days a week. The group that were slightly younger (by a few months) were nightmarishly difficult to control. The slightly older group were angelic.
The best advice already given that I wanted to stress was developing a routine. I broke the lesson down into short segments of action and quiet time like this:
*15 minutes of song and dance (a few old songs and a new one every few days)
* 5 minutes of language review (usually with flashcards)
* 10 minutes of new language presentation (using one or two sit down games)
* 10 minutes of language practice using a game with movement
*break time - 10 minutes
* 10-15 minutes of storybook and activities (our school curriculum had one new book per week for us, by the end of the week they could often repeat many parts of the whole story, if not the whole thing)
* 5 minutes of stretching/ exercise or Simon says type activity to get the blood flowing again
* 10-15 minutes of colouring/ worksheet, often while playing the songs from period one or another English tape
* clean up time with the clean up song
It sounds fairly rigid, but of course you have to read the class's spirit of the day to gauge the timing of each segment. At first I thought 80 minutes would be impossible, but with the breakdown time went really fast.
It's true variation is good, but repetition is also good. One day one little guy saw me point to the clock and he
exclaimed "Look at the time!"
which I then realized was something I must have been saying all the time without noticing.
Anyway, good luck with your class. This thread
has a bunch of activities you can use to fill in the time slots.