Final answer:
The most appropriate measure to ensure a child's ET tube is correctly positioned is to monitor for a yellow display on a CO2 monitor, which indicates the presence of exhaled carbon dioxide and confirms proper placement.
Step-by-step explanation:
In ensuring that a child's ET (endotracheal) tube is correctly positioned, the nurse should monitor for a yellow display on a CO2 monitor as a key indicator of proper tube placement. This yellow display indicates the presence of exhaled carbon dioxide, confirming that the tube is in the respiratory tree rather than the esophagus. Other options such as auscultating for abdominal breath sounds, marking the tube at the lip, and inspecting for water vapor in the tube can assist; however, the CO2 monitor provides a more definitive and immediate assessment of correct intubation. Furthermore, techniques such as auscultating the lungs with a stethoscope for breath sounds, checking blood oxygen levels with a pulse oximeter, and using chest X-rays can all provide additional information on both correct placement and overall pulmonary function.