Final answer:
The child's return to diapers while hospitalized signifies normal regression due to the stress of the hospital environment and is a temporary coping mechanism, not a reflection of developmental issues or the appropriateness of prior potty training.
Step-by-step explanation:
The nurse should reassure the parents that the 30-month-old child reverting to diapers while hospitalized is experiencing a normal behavior known as regression. This phenomenon is common among young children who are faced with stressful situations or significant changes to their routine, such as a hospital stay. It does not indicate developmental delays or urinary urgency specifically due to the hospitalization, nor does it suggest the child was too young to be potty-trained. Instead, it's a temporary coping mechanism that children sometimes display when they're under stress. It is a part of the child's psychological response to the unfamiliar environment and potential anxiety related to the hospital experience.