Regardless of the age you teach, your students are looking to you to be competent and confident. Even if you may not feel confident on some days, it is important that you show it.
Confidence from the teacher comforts students. They need to have a feeling that you know what you are doing and that you are in control. If you are wishy-washy or uncertain, you may lose their respect.
This does not mean that you have to be perfect, or feel bad if you aren't perfect. It's not the end of the world if you make a mistake every now and then. That is much different than not showing a general vibe of confidence, though.
Believe in yourself and what you are doing. Don't freak out if you mess up occasionally. Just be sure to keep your swagger.