Final answer:
The priority for a nurse caring for a toddler with gastroenteritis due to Salmonella is to initiate contact precautions to prevent the spread of infection.
Step-by-step explanation:
The priority action for a nurse caring for a toddler with gastroenteritis caused by Salmonella is to initiate contact precautions. This is essential in preventing the spread of the infection to others, as Salmonella is highly contagious. Immediate measures should be taken to ensure that the child is in a safe environment that minimizes the risk of transmission. While obtaining a recent food history, establishing a skin care routine, and weighing the child are important, they are secondary to preventing the infection's spread.