This paper describes a course attended by 30 graduate students in mathematics education, nonpreschool teachers, that aimed to promote participants' knowledge of young children's development of numerical competencies. Prior to the course, participants held high positive beliefs towards children's engagement with numerical activities, but their self-efficacy was low. Findings indicated that mastery experience, mostly by repeatedly analyzing videos, and repeatedly designing tasks, afforded participants a chance to see how they were progressing and increased their self-efficacy.