The sentences that demonstrate correct subject-verb agreement are:
- The children visited their grandparents last weekend.
- They visited their grandparents last weekend.
Both of these sentences use the past tense verb "visited" correctly with the plural subjects "children" and "they."
The other sentences, "The children visit their grandparents last weekend" and "They visit their grandparents last weekend," incorrectly use the present tense verb "visit" with a time expression ("last weekend") that requires the past tense.