Objective Lacosamide (LCM), a newly approved third-generation antiepileptic drug, is still considered a novel therapy. Recommendations for adjunctive LCM treatment in pediatric epilepsy are limited, and factors influencing its efficacy remain unclear. Therefore, the therapeutic effects of LCM in children require further large-scale investigation. To evaluate the real-world efficacy, safety, and tolerability of LCM in a pediatric population with broad-spectrum epilepsy through a prospective study. Methods This prospective observational cohort study enrolled children aged 4–18 years with epilepsy who visited Qingdao University Affiliated Women and Children’s Hospital between January 2021 and January 2024. Participants included those with focal, generalized, unknown-onset epilepsy, and complex syndromes such as Lennox-Gastaut syndrome (LGS), febrile infection-related epilepsy syndrome (FIRES), and infantile spasms. Baseline seizure frequency and clinical characteristics were recorded prior to initiating LCM. LCM was administered at the starting dose recommended by the package insert and titrated to a maintenance dose based on efficacy and tolerability. Follow-up assessments occurred at weeks 12, 24, and 36, evaluating seizure frequency, concomitant antiepileptic drug use, and adverse events. The primary outcomes were treatment response (≥50% reduction in seizure frequency) and seizure-free rate, while secondary outcomes included incidence of adverse events and drug retention rate. Data were analyzed using self-controlled comparisons, and logistic regression was applied to explore clinical factors affecting efficacy. Results The overall retention rates at weeks 12, 24, and 36 following LCM adjunctive therapy were 98.0%, 97.0%, and 95.0%, respectively. Corresponding response rates were 79.2%, 78.8%, and 75.5%, and seizure-free rates were 54.5%, 56.6%, and 57.1%. Statistically significant differences (P<0.05) between responders and non-responders were observed in baseline seizure frequency, seizure duration, number of concomitant antiepileptic drugs, presence of developmental delay/intellectual disability, and normality of cranial MRI findings. Logistic regression indicated that the number of concomitant antiepileptic drugs and normal cranial MRI findings were independent factors influencing LCM efficacy. Adverse events included dizziness, somnolence, blurred vision, irritability, and headache, with an overall incidence of 6.9%. Conclusion Adjunctive LCM treatment in children aged 4~18 years demonstrates high efficacy and seizure-free rates, mild adverse effects, and favorable retention. LCM appears particularly effective in patients with normal cranial MRI findings and limited concomitant antiepileptic drug use (≤2 drugs).