Anti-Bacterial Agents/therapeutic use; Appendectomy/methods; Appendicitis/drug therapy; Appendicitis/surgery; Cellulitis/drug therapy; Cellulitis/surgery; Acute Disease; Appendicitis/diagnostic imaging; Boston; Cellulitis/diagnostic imaging; Child; Child, Preschool; Cohort Studies; Female; Follow-Up Studies; Hospitals, Pediatric; Humans; Infusions, Intravenous; Male; Monitoring, Physiologic/methods; Postoperative Complications/physiopathology; Postoperative Complications/therapy; Preoperative Care/methods; Retrospective Studies; Risk Assessment; Severity of Illness Index; Time Factors; Tomography, X-Ray Computed/methods; Treatment Outcome