A 5 y/o F weighing 20 kg presents to the Emergency Department complaining of abdominal pain. She is alert and nontoxic appearing. She does have a slight tachycardia and a temperature of 38.1. Her BP and RR are within normal limits. She is tender in the RLQ and you suspect appendicitis. She was given Acetaminophen 300 mg PO and Ibuprofen 200 mg PO at triage but is still in moderate pain while awaiting ultrasound. Which of the following is the BEST option for pain management for her now?