Here's why:
* Proper nouns are specific names of people, places, or things. They are capitalized. For example: John, London, Mount Everest.
* Common nouns are general names of people, places, or things. They are not capitalized unless they are at the beginning of a sentence. For example: boy, city, mountain.
"Children" is a common noun because it refers to a general group of people. It's not a specific name.