Trajectories of Change, Illness Understanding, and Parental Worries in Children and Adolescents Undergoing Internet-Delivered Cognitive-Behavioral Therapy for Functional Abdominal Pain Disorders: Protocol for a Single-Case Design and Explorative Pilot Study - Eva Skovslund Nielsen; Karen Kallesøe; Tine Bennedsen Gehrt; Ellen Bjerre-Nielsen; Maria Lalouni; Lisbeth Frostholm; Marianne Bonnert; Charlotte Ulrikka Rask (JMIR Research Protocols)