A 3-year-old male with known glucose 6-pyruvate dehydrogenase (G6PD) deficiency presents to the emergency room with difficulty breathing and blue lips. Per the mother, the child recently had topical local anesthetic cream applied to his arms in preparation for lab draws. The pulse oximeter is reading 86%, but his PaO₂ is normal. Which of the following is the BEST initial treatment for this child?