The sentences that demonstrate correct subject-verb agreement are:
- "The children visited their grandparents last weekend."
- "The children visit their grandparents last weekend."
The other options are incorrect because:
- "They visit their grandparents last weekend" is incorrect because it mixes present tense ("visit") with a past time reference ("last weekend").
- "They children visited their grandparents last weekend" is incorrect because "They" and "children" do not agree; "They" should not be used with "children" in this context.
Therefore, the correct responses are the first and third sentences.