i-CBT Functional Gastrointestinal Disorders in Youth: the Impact of Negative Illness Understanding and Parental Illness Worries

Inclusion Criteria: * Child age: 8-12 years, adolescents: 13 - 17 years * A primary diagnosis according to the ROME-IV criteria of one of the FGID sub-types: Irritable bowel syndrome (IBS) or functional abdominal pain not-otherwise-specified (FAP-NOS) * The diagnosis should be documented by their regular physician in the somatic setting, and recommended routine medical investigations should be evaluated as normal or without clinical significance (include growth; blood samples including TSH, total IgA, IgA tissue transglutaminase, complete blood count, C-reactive protein analysis, liver enzymes; and fecal calprotectin) * Stable dosage of FGID-related medication such as laxatives, anti-diarrheal medication or pain-modulating psychopharmacological medication during the past month. Exclusion Criteria: * Another disease that explains the symptoms; * Severe psychiatric or social problems (e.g., high level of suicidal ideation or ongoing abuse); * Ongoing psychological treatment; * Insufficient language or computer skills (patients and parents); * Severe family problems (e.g. child abuse, parental substance abuse or severe psychiatric illness, ongoing custody fight) * School absence of more than 40% over the past month.