You should use your words to describe what you want them to do while avoiding the words public speaking. The reason for this is the fact that the term public speaking brings over whelming feelings of nervousness to a child. It can scare them. Instead make it fun by calling it something else. Idea day or Share your knowledge day can give them confidence and make them feel like they are in the lead role.
Give them topics that they think that they know well. By making they research something that they think that they know you open another angle. Not only do they have to really understand their topic, but they also have to know how to express their ideas to others. If they are passionate about something it will carry over into the speech they write. Passion will equate into strong ideas.
Start them off in more intimate groups will allow them a chance to build confidence. A child that is giving an opportunity to build confidence will be more outspoken about their topic. As they get comfortable speaking increase the number in their audience. Soon, they will see a large crowd is nothing to be nervous about.
Most importantly, teach them the basics of writing notes and creating an outline. This is what is important, not their technique. As long as they can clearly gather their ideas, the technique will come. Patience and understanding will help you teach even the most shy child to stand up and speak about a topic in a clear and efficient way.