To assess the effectiveness of an analgesic for a 4-year-old child, what intervention should the nurse implement?

Using a happy-face/sad-face pain scale is particularly effective when assessing pain in young children, as it utilizes visual cues that are age-appropriate and easily understood by preschoolers. This type of scale allows children to express their discomfort in a way that they can relate to, making it easier for the nurse to gauge the child's pain level before and after administering the analgesic.

The other interventions, while potentially useful in other contexts, aren't as suitable for a 4-year-old. Asking the mother for her opinion on the effectiveness of the analgesic may not accurately reflect the child's own experience of pain. Changes in vital signs can indicate stress or discomfort but are not specific indicators of pain relief or assessment. Teaching a child to use a numeric pain scale would likely be beyond the cognitive abilities of a 4-year-old, as this age group typically doesn't yet have the conceptual understanding required to use numeric scales effectively. Therefore, the happy-face/sad-face pain scale is the most appropriate choice to assess pain in this situation.