Find out how old the children are. If they are very young, they probably won't be able to understand big words or listen to you for long. The younger the children, the simpler the speech should be. Find out what kinds of experiences connect each of the children. Perhaps they are in a band class together and all share a love of music, or in an art class and enjoy painting. Whatever interests them will hold their attention the longest and that is what you should focus the topic of your speech on.
Follow the typical structure of other types of speeches. There should be a beginning, a middle and a conclusion. Use simple language and personalize your speech so that it reads as a story. Focus on just one or two points you want to talk about. Use words that are fun and interesting. Provide opportunities for the children to participate during your speech. Ask questions and let them ask questions. When you are finishing your speech, tie it back to the things you said at the beginning.
Include specific examples. Whenever you can, show your audience what you mean -- whether through body language, or by acting out different scenes or even just by using different voices. Use props to make your speech more interesting. When you are done writing your speech, practice reciting it for adults and take their feedback into account. Modify your speech as necessary to make sure it is perfect.