Being able to debate and argue a point is a skill that is valuable in many aspects of life. It is even beneficial for children to learn this kind of skill at a young age. This allows them to only become better debaters as time goes on and helps them learn how to accurately express their thoughts and opinions.
Teach children how to create and support an argument. If they are young, it is probably best that you have them work with a position that they agree with rather than assigning a topic and a position. Take an easy "right or wrong" topic like fighting. Ask the child if they think it's right or wrong and then ask them to write down why they feel that way.
Assign main points. The best way to make any debater feel confident is to make them feel prepared. You should have the child come up with all the reasons why they believe their position is right and then to pick the best and most important from them. Have them put their arguments in order starting with the one which they feel strongest about. Explain that they need to write their point out as a statement and then explain why with supporting evidence.
Create opposing statements by having the child list reasons why their position may be wrong to other people. For example, if they are arguing against fighting, have them consider situations where they might find this behavior acceptable. This will bring up some opposing arguments like self-defense. This helps prepare them to respond to arguments and can even help to make their main points stronger.
Teach body language by having the child argue their cause while facing you. Emphasize that they should always keep eye contact and maintain a good posture. Let them know that occasional hand gestures are encouraged, and that they should avoid keeping their hands by their sides.
Teach the child how to project their voice and how to properly incorporate pauses in their speech. Discourage long sentences and fast speaking and reward loud voices and confidence.