Introduction: In this study, we prospectively investigated changes in serum interleukin-6 (IL-6), high-sensitivity C-reactive protein (hsCRP) and full white blood cell (WBC) counts during the diagnosis and treatment of paediatric patients with appendicitis. We also investigated the effects of the COVID-19 pandemic on the diagnosis and treatment processes of paediatric appendicitis patients. Materials and Methods: A non-perforated appendicitis group ( n = 110), a perforated appendicitis group ( n = 35) and an appendicitis + COVID-19 group ( n = 8) were formed. Blood samples were taken upon admission and every day until the three studied parameters returned to normal values. To investigate the effects of the COVID-19 pandemic on paediatric appendicitis patients, the perforated appendicitis rates and the times from the onset of the first symptoms to the operation before and during the pandemic were compared. Results: WBC, IL-6, and hsCRP dropped below the upper limits on the second postoperative day in the non-perforated appendicitis group, four to six days postoperatively in the perforated appendicitis group, and three to six days postoperatively in the appendicitis + COVID-19 group. These parameters were not within normal range in patients who developed complications during follow-up. The time from the onset of abdominal pain to the surgery was significantly longer during than before the pandemic in both the non-perforated appendicitis group and the perforated appendicitis group. Conclusions: Our results show that WBC, IL-6, and hsCRP are useful laboratory parameters that can complete clinical examinations in the diagnosis of appendicitis in paediatric patients and the identification of complications that may develop postoperatively.