Step 1: Understanding Reflexive Pronouns:
When the subject and the object of the sentence are the same, we use reflexive pronouns like "themselves." In this case, the children are locking themselves up, so "themselves" is the correct reflexive pronoun.
Step 2: Analyzing the Options:
- Their: Incorrect because "their" is a possessive adjective, not a reflexive pronoun.
- Himself: Incorrect because "himself" is a reflexive pronoun used for masculine singular subjects. Here, "children" is plural.
- Them: Incorrect because "them" is an object pronoun, not a reflexive pronoun.
- Themselves: The correct reflexive pronoun for plural subjects, like "children."
Step 3: Conclusion:
The correct pronoun is themselves, as it refers back to the plural subject "the children."