Background To determine if non-operative treatment is safe in children with acute appendicitis, we evaluated the incidence of unexpected findings after an appendectomy in children, and the influence they have on subsequent treatment. Methods A historical cohort study (January 2004–December 2014) was performed including children, aged 0–17 years, who underwent an appendectomy for the suspicion of acute appendicitis. Patients were divided based upon histopathological examination. Unexpected findings were reviewed, as well as the subsequent treatment plan. Results In total 484 patients were included in this study. In the overall group, unexpected findings were noted in 10 (2.1%) patients of which two patients intra-operatively with a non-inflamed appendix (Ileitis terminalis N = 1 and ovarian torsion N = 1) and in 8 patients on histopathological examination. The latter group consisted of 4 patients with concomitant simple appendicitis (parasitic infection N = 3 and Walthard cell rest N = 1), two with concomitant complex appendicitis (carcinoid N = 1 and parasitic infection N = 1) and two patients with a non-inflamed appendix (endometriosis N = 1 and parasitic infection N = 1). Treatment was changed in 4 patients (< 1%). Conclusions Results from this study corroborate the safety of non-operative strategy for acute simple appendicitis, as the occurrence of unexpected findings was low, with extremely few necessary changes of the treatment plan because of serious findings. Type of study Prognosis study. Level of evidence Level 2 (retrospective cohort study).