A child (the plural is children) is a person below the age of puberty (biologically) or below the age of majority (legally). A person can be called his or her parent's child, no matter how old he or she is, a synonym for the word 'child' is kid.
In rich countries, children go to school. Small children may go to nursery school, preschool or kindergarten. Playing with small toys and using their imaginations are important for child development. But often in a developing country, children sometimes work in factories or in the fields with their parents or guardians.
When two people get married and one (or both) of them have children with someone else before this marriage, that child is called a step child by the child's non-birth parent. If both people had children, the children of each parent are step brothers or step sisters of each other. A child whose parents have died is an orphan. Orphans with no parent may be raised by grandparents or other relatives. Otherwise they may go to live in an orphanage. They may be adopted by another adult or couple wanting a child. They will then be able to grow up in a new family.