The sentences that demonstrate correct subject-verb agreement are:
- They visited their grandparents last weekend.
- The children visited their grandparents last weekend.
Both use the past tense "visited," which agrees with the plural subjects "they" and "the children."
The sentence "The children visit their grandparents last weekend." and "They visit their grandparents last weekend." are incorrect because they use the present tense "visit" with a time expression "last weekend," which refers to the past.