A standardized severity assessment approach is needed in children with appendicitis for postoperative adverse event estimation and severity adjustment for hospital-level comparative performance reporting. To examine the association between the presence and number of National Surgical Quality Improvement Program (NSQIP) Pediatric-defined intraoperative criteria for complicated appendicitis and outcomes in a population-based sample of children. This cohort study used data from the American College of Surgeons NSQIP Pediatric Appendectomy Procedure Targeted Participant Use Data File and General Participant Use Data File for children younger than 18 years who underwent appendectomy from January 1, 2019, through December 31, 2022, at 148 hospitals participating in NSQIP Pediatric. The presence of NSQIP Pediatric intraoperative criteria for complicated appendicitis (ie, visible perforation, intraperitoneal abscess, extraluminal fecalith, and diffuse fibrinopurulent exudate). Adverse event outcomes included postoperative rates of any surgical site infection (incisional or organ space), percutaneous drainage, sepsis, and reoperation. Resource use outcomes included operative duration and hospital length of stay, and rates of postoperative imaging, parenteral nutrition use, and revisits. Multivariable regression was used to explore the influence of individual and combinations of intraoperative criteria on outcomes after adjusting for patient characteristics. Of 82 950 patients included, 23 221 (27.9%) had at least 1 finding of complicated appendicitis. Compared with cases without any criteria present, the presence of each finding of complicated appendicitis was independently associated with higher rates of any adverse events; adjusted odds ratios (AORs) by finding were 5.57 (95% CI, 5.04-6.15) for visible hole, 4.83 (95% CI, 4.17-5.59) for diffuse fibrinopurulent exudate, 7.06 (95% CI, 5.77-8.63) for abscess, and 6.62 (95% CI, 4.78-9.15) for fecalith. An increasing number of criteria was associated with a stepwise increase in risk of any adverse events; AOR by number of criteria met were 5.55 (95% CI, 5.09-6.05) for 1 criterion, 8.86 (95% CI, 8.16-9.62) for 2 criteria, and 16.65 (95% CI, 15.10-18.35) for ≥3 criteria. Similar patterns in criteria-specific and cumulative implications for outcomes were observed with each individual adverse event and resource use measure. This cohort study found that postoperative complications and increased resource use are associated with the presence and number of NSQIP Pediatric criteria for complicated appendicitis. These criteria should be considered the gold standard, evidence-based severity assessment framework for estimating risk of adverse events and resource use in children with appendicitis.