Here is a technique that can help you to build basic habits for your kid to become a lifelong strong public speaker. It is one that i use with my own daughter. I make sure that she gets to speak up and interact with people whenever she can.
For example, when she wants something at a restaurant, i do not simply order it for her. Instead, i encourage her to ask the waiter or waitress for it herself. She can be a little shy but eventually she looks the waiter in the eye and asks for water or juice or whatever it is she wants in a bold manner. It may not be the biggest speech in the world, but it is an important step to take.
The basic principle that i am trying to instill here is that it is okay to speak to someone in public when there a lot of people around. I want her to realize that if she speaks loud enough that people hear and understand her, then she can influence them. The good thing about a waiter is that if you tell them what you want, they will bring it to you. So it is a good learning process.
Do i ever get in a hurry and order for her sometimes? Sure. But if we have time then ordering her own drink or what she wants for food is a very simple action that helps her build a skill at an early age.