A 10-year-old girl diagnosed with ADHD (inattentive type) was started on methylphenidate 10 mg once daily 4 weeks ago. Her mother reports modest improvement in attention at school, but the child now complains of decreased appetite and mild insomnia. Vital signs show a heart rate of 92 bpm (baseline 78 bpm) and blood pressure 108/68 mmHg (baseline 102/64 mmHg). The child is otherwise well, and no new comorbidities have emerged. What is the most appropriate next step in management?