What is the use of child classes?

Answer 1

Child class is another name for derived class.
And the base class is also known as the parent class. It's called inheritance.
If you want to know more about inheritance read the course.

Answer 2

child classes are for doing something in a certain order, child are the last ones to be executed. for example if you wanted to make an error output, first the error shows up an then weathered else you have