Here's why:
* Passive voice is used when the subject of the sentence is the receiver of the action, rather than the performer.
* In this sentence, the action is "being tired" and the subject is "children." They are the ones experiencing the tiredness, not the ones causing it.
To understand the passive voice better, let's compare it to the active voice:
* Active voice: "The day tired the children." Here, "day" is the subject and it's performing the action of tiring.
* Passive voice: "By end of day children were tired." Here, "children" is the subject and they are receiving the action of being tired.
While the sentence is grammatically correct, it's often preferred to use the active voice for clarity and conciseness.