First of all, you must know what you want to teach the kids. You can write the lessons on a piece of paper. For instance, you want to teach the kids that they should not fight with each other or they should behave with each other in a good manner.
After knowing what you want to teach you kids. You have to start making an outline of your speech. You must know that the outline contains the start, middle and end of your speech. During the start, you have to tell the kids about yourself. You should introduce yourself in a way that they may understand you. Use simple words and short sentences. After introducing yourself, you have to make the main part of your message in the middle of your speech. Note important points of your main message which you want to convey to the kids. Finally, you have to note down the core points with which you will end your speech.
After making the outline, start describing each point in it. While writing the description, you must know that you are going to address kids. Therefore, you have to use a very simple language.
Besides, you should include short stories to make your speech sound good to the kids. You can also add humorous material in your speech to make the kids feel comfortable and happy.