The correct answer is young children.
A subject of a sentence is a word or phrase that does the action within that particular sentence. In the example above, the subject may know more about life.
There is a difference between a simple subject and a complete subject, however. A simple subject is one word only, whereas a complete subject would include modifiers as well. In the sentence above, the simple subject would be children, but the complete subject is young children (the adjective included).
