Automatically Create a New Child List

You can create a new child list automatically when creating a new Associated Items column.

Simply enter a new for the column, a title for the new list and the type of relationship you need (content type or lookup).

Once you click on Save the process will generate a new list with the name you provided. The only column in this new list will be Title. You will need to add your other columns for the child and the view when designing the form and child list view. 

This is the easiest and fastest way to create new associated items. However, the manually approach is equally valid, especially for cases when the child list already exists.