I have the same fear even now (especially for presentations). Sometimes I get jitters when I start speaking before everyone in my classroom. However, I have it well under control using the following methods.
Good preparation is the key to avoid public speaking fear.
I also practiced speaking before the mirror. Through this way, I can see my facial expressions on my own and I will be rest assured with it. You can encourage children to do so. When you try this a few times in home, you will be more relaxed during the presentation.
Teach breathing techniques for children. Check out the term 'belly breathing' and how it is practiced. When you do this before the presentation, your muscles will be more relaxed. Children practicing this from a young age can better themselves in public speaking.