Use of Tapentadol Oral Solution for Pain After Surgery in Children From Newborn to Less Than 2 Years Old

This clinical trial has 3 phases: enrollment, treatment (15 hours) and follow up. During the enrolment phase consent and eligibility will be determined. After surgery, the participant will be given routine pain medication as per standard of care in the hospital. Treatment phase: When the participant has a functioning gastrointestinal tract after surgery, can tolerate medication administered orally or via a feeding tube, meets the inclusion criteria, and does not meet any exclusion criterion, the participant will be allocated to the investigational medicinal product (IMP). Evaluations will be performed over the next 15 hours, including the assessment of the amount of pain. During this time, 2 blood samples will be taken for testing of the amount of tapentadol and its main metabolites in the participant's blood. A final follow-up visit is planned to take place up to 2 weeks after taking the trial medication.