The sentences that demonstrate correct subject-verb agreement are:
- They visited their grandparents last weekend.
- The children visited their grandparents last weekend.
Both sentences correctly match the plural subject ("They" and "The children") with the past tense verb "visited."
The other sentences ("The children visit their grandparents last weekend" and "They visit their grandparents last weekend") are incorrect because they imply an ongoing or habitual action that does not match the time frame indicated by "last weekend."