Final answer:
In the sentence provided, "a cup of tea" is the object because it is receiving the action performed by the subject, "the children," who drank it.
Step-by-step explanation:
In the sentence "The children drank a cup of tea," the object is "a cup of tea." This is because it is the thing that is being acted upon by the subject, which in this case are "the children." The children are the ones performing the action (drinking), and "a cup of tea" is what they consume. In English grammar, the object is the entity that is acted upon by the subject, and it usually follows the verb within the sentence structure.