Creating a child Runbook is one of the most essential tasks in SCO. Technically, there is no difference between normal Runbooks and a child Runbook.
A child Runbook is a Runbook with initialized data and often return data and is mostly used for tasks that are needed by multiple Runbooks.
For example, a Runbook, which is adding a user in the AD Group can be a child Runbook, as this task is often used by different Runbooks.
So, it makes sense to build one Runbook, such as a module PowerShell programming, with all the features you need, instead of building the task in each Runbook separately.
See this chart for a better understanding of how a child Runbooks works: