20 kids is nothing, try 70. The truth is with kids that young you have to try and throw in a bit of entertainment. When I first started teaching kindergarteners I found that exaggerated body movements when using flash cards, showing them too fast for the students to see, hiding them and asking for an answer, all these kinds of things worked wonders. Kids this age will not have their attention held with mere talking alone. You have to move a lot, change tone of voice quite a bit, use a lot of visual aids, make what you are doing simple, change activities constantly, and play games with them. Games of course meaning take something educational, and make it into a game. Children that young have very short attention spans, and may only be able to focus on one thing for 3-5 minutes at a time. The older they get, the longer their attention can be held. For children this age, for a 45 minute class, you'll need to plan at least 7 separate activities for each class.

This is one of the reasons why kindergarten teaching jobs are the "least" sought after, and most hated teaching jobs for foreign teachers. The children are a pain in the butt, have little attention span, and can act up quite a bit. I had minimal problems, but I understood these simple little things that can make a huge difference. Change your presentation style, use a lot more body language, and increase the number of "separate" activities in your class. YOU can't keep going in your class from start to finish and move from one thing to the next without doing so in a clear, stark way. Kids get bored easy, and want to move on to new things quickly. If they don't actually "know" there is something new going on, they may not realize it even if something new is going on. That's why, in addition to everything else I mentioned, I am clear in ending one activity, and very clear in letting them know we are moving on to a new one.

You also have to have a firm hand, and can't be afraid to be strict. Not too strict, as they are only 5, but you have to be firm.

Hope this has been of some small help. There is more I could say, but this will hopefully suffice.