I have a question.
What is the difference between entre kid, boy, child, children
When is it used?
A kid is any child, either a boy or girl or a baby goat. It's slang but in common usage.
A boy is a male child probably ten years or under, but not always. "The boys are going to the bar." It's a slang for men getting together to have fun. It's casual not literal. (You can substitute girl/s for boy/s and female for male.)
A child is a person, male or female, usually under ten, but it's flexible too. My 16 year old is still my child but she is not a child.
Children is simply the plural (more than one) of child.