Background: Gingivitis is the most common periodontal disease in children and adolescents, characterized by reversible inflammation caused by plaque accumulation. Because traditional clinical indices, such as gingival and plaque scores, detect inflammation only after visible tissue changes occur, identifying early biomarkers of subclinical inflammation is essential. Gingival crevicular fluid (GCF), a serum-derived exudate enriched with host inflammatory mediators, has emerged as a valuable non-invasive diagnostic medium. This study aimed to compare GCF concentrations of interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and interleukin-8 (IL-8) between children with clinically healthy gingiva and those with plaque-induced gingivitis, and to evaluate correlations between cytokine levels and clinical periodontal parameters. Methods: A cross-sectional study was conducted in 58 children aged 10–13 years, classified as healthy (n = 30) or with gingivitis (n = 28). Clinical parameters—including plaque index, gingival index, probing depth, bleeding on probing, and GCF volume were recorded. GCF samples were collected from the mesiobuccal sites of the upper incisors and analyzed using enzyme-linked immunosorbent assay (ELISA). Statistical tests included Mann-Whitney U, Spearman correlation, and receiver operating characteristic (ROC) curve analysis (p < 0.05). Results: All clinical parameters were significantly higher in the gingivitis group (p < 0.001). IL-1β and IL-8 levels were significantly elevated in gingivitis (p = 0.001 and p = 0.035, respectively), whereas IL-6 and TNF-α showed no significant differences. IL-1β correlated with bleeding on probing (r = 0.47) and pocket depth (r = 0.34). IL-1β demonstrated the highest diagnostic accuracy, with an area under the curve (AUC) = 0.76. Conclusions: IL-1β appears to be a sensitive and reliable biomarker of gingival inflammation in children, reflecting local immune activation before irreversible tissue damage.