Acute appendicitis is common in children and adolescents. Recently, conservative antibiotic treatment is regarded to be a safe approach to treat uncomplicated appendicitis. It is already established as initial treatment in cases of perforated appendicitis with perityphlitic abscess, commonly followed by interval appendectomy. We report on a 13-year-old boy with uncomplicated appendicitis and a 17-year-old girl with complicated, perforated appendicitis and perityphlitic abscess in whom initially successful antibiotic treatment led to a delay in detection of a carcinoid tumor (neuroendocrine tumor, NET) of the appendix. NET of the appendix, with an incidence of 0.03 to 0.8% in the pediatric population undergoing appendectomy for acute appendicitis, are usually incidental findings after appendectomy with no secure method for detection prior to surgery. Raising concern about this rare but severe disease, we recommend information of patients and their parents about the potential risk of belated diagnosis before opting for conservative their treatment of acute appendicitis. Furthermore, patients successfully treated conservatively require a close follow-up by ultrasound. In presence of any conspicuous finding, especially on imaging, appendectomy should be considered.