A child is a person under the age of 18, but there are many different stages a person goes through before becoming an adult. A baby is under the age of one. A toddler is between the ages of one and two. A preteen might be ten or eleven. A teenager is between the ages of 12 and 19. These are all children (plural).*
She’s a beautiful child.
The child’s bedroom is full of stuffed animals and toys. (The word "child" is singular and possessive in this sentence.)
What were you like as a child?
Did you have a good childhood? (childhood = time during which a person is a child)
The word "child" suggests a lower level of maturity, so it can be used to describe adult activity:
Bill is a man-child. He’s 32 years old, lives with his parents, likes to play video games, and avoids positions of responsibility.
Gabrielle throws temper tantrums and acts like a child when things don’t go her way. (This can be said of a child or an adult.)
This game is child’s play. (It’s easy.)
The plural form of child is children.
Do you have any children?
How many children do you have?
Jennifer has one child.
Her sister, Tracy, has two children.
The children’s play area is monitored by an adult. (Note that the possessive form of "children" uses " ‘s."
Leslie enjoys reading children’s books to her son.
The word "child" is often used with other nouns to form compound nouns:
Child development is an area of academic study in which students learn how children mature.
Child care is provided when a mother and/or father goes to work during the day and needs someone to take care of the kids.
Child abuse is a situation in which a child is regularly beaten by a parent or a custodian.
Child support is paid to a spouse who is divorced and receives financial support as a condition of ending the marriage. Usually a man pays child support, but it’s also possible that a woman who makes a lot of money would pay child support to an ex-husband.