The pediatric population is a vulnerable group for adverse drug reactions (ADRs), and data on spontaneous reporting of ADRs in the hospital setting are scarce. We conducted a retrospective analysis of ADRs in pediatric patients spontaneously reported by health care professionals to a Pharmacovigilance Program in a tertiary hospital between 2010 and 2020, and we compared characteristics of ADRs between pediatric age subgroups. From 1787 spontaneously reported ADRs in an 11-year period, 103 (5.85%) were pediatric ADRs. The median age of patients with ADRs was 8.4 years (range 1 day–17 years) and 57.3% were male. The most frequent ADRs reported were nervous system disorders (13.6%) and the most frequently involved drugs were antineoplastics and immunodulators (32.4%). A 59.2% of the ADRs were serious and 55.3% were classified as being type B reactions. Medication errors were involved in 7.8% of the ADRs and 11.9% of the suspected drugs were used off-label. Spontaneous reports of ADRs in newborns, infants, and toddlers were more serious and less often described in the product data sheet than in children and adolescents (p < 0.001 and p = 0.004 respectively). Medication errors were more frequent in patients under two years of age. These results should be interpreted with caution due to under-reporting and biases in spontaneous reporting of ADRs.