A 6-month-old boy presents to the pediatric emergency department with a 2-week history of brief, sudden flexion of the head, trunk, and arms occurring in clusters, especially on awakening. The mother reports these episodes happen 5–10 times daily and are often mistaken for startle or gastroesophageal reflux. EEG shows the pattern marked **D** in the diagram. Which of the following is the MOST appropriate FIRST-LINE management for this condition?