The direct object in the given sentence is "ball."
To identify the direct object in a sentence, you need to identify the verb and ask "what" or "whom" is being directly affected by the action of the verb. In this sentence, the verb is "kick" or "head." So, you can ask "what" is being kicked or headed by the players. The answer to that question is "the ball." Therefore, the direct object in this sentence is "ball."
