Your job as the teacher is to help children to be comfortable and successful in their church school experience. If you can be flexible and open to the children's responses, they will have a good experience. Your positive attitude goes a long way toward empowering the children to participate fully.
Sometimes you will have to go beyond your own comfort zone with the arts to encourage the children to do so too. If you are personally challenged by arts experiences, practice at home until you are comfortable with the process you have chosen to lead. Think of yourself as a guide into the materials and process rather than as an an expert. Bring spontaneity and playfulness to the class.
You may be an expert in some arts techniques and have difficulty getting away from professional expectations for the children. See art as communication and as a method for expressing spiritual experiences, not just as a lesson in techniques. Let the arts slow you down to be present with the process rather than the outcome. Don't only lead what you are comfortable in; take a chance so that the children will too.
Make sure that you talk to your co-teacher or assistant about the process of using art spiritually. Between the two of you, you both need to be able to approach the arts activities in new and positive ways. Children need to be invited into the process even they don't feel particularly creative