Final answer:
The necessary precaution for children using fluoridated toothpaste is to be supervised to prevent swallowing toothpaste.
Step-by-step explanation:
The necessary precaution for children who are using fluoridated toothpaste is c) They should be supervised to prevent swallowing toothpaste. Fluoride in toothpaste is beneficial for dental health, but swallowing large amounts can be harmful, especially for young children. Therefore, it is important for parents or guardians to supervise children while they brush their teeth to ensure they spit out the toothpaste and don't swallow it.