Compared to adults, severe or fatal COVID-19 disease is much less common in children. However, a higher risk for progression has been reported in infants. Different pediatric COVID-19 severity scores are reported in the literature. Methods: Subjects under 90 days of age admitted to 35 Italian institutions for COVID-19 were included. The severity of COVID-19 was scored as mild/moderate or severe/critical following the classification reported in the literature by Venturini, Dong, Kanburoglu, and Gale. To assess the diagnostic accuracy of each classification system, we stratified all enrolled patients developing a posteriori severity score based on clinical presentation and outcomes and then compared all different scores analyzed. Results: We included 216 infants below 90 days of age. The most common symptom was fever, followed by coryza, poor feeding, cough, and gastrointestinal manifestations. According to Venturini, Dong, Kanburoglu, and Gale’s severity scores, 18%, 6%, 4.2%, and 29.6% of infants presented with severe/critical disease, respectively. A correlation analysis between these four scores and the a posteriori severity score assigned to all enrolled subjects was performed, and a crescent strength of correlation from Gale (R = 0.355, p < 0.001) to Venturini (R = 0.425, p < 0.001), Dong (R = 0.734, p < 0.001), and Kanburoglu (R = 0.859, p < 0.001) was observed. Conclusions: The percentage of infants with severe COVID-19 varies widely according to the score systems. A unique clinical score should be designed for neonates and infants with COVID-19.