Enter a separate column for the Parent Matrix Item and another column for the Child Matrix Item on the CSV file.
The CSV file will look like this:
The above example assumes that the Department field is being updated on the Child Matrix Item record.
Also, under the Item Name/Number column, we need to enter the Child Matrix Item Name only, without including the Parent Matrix Item's name. (e.g. Use Widget rather than Parent Widget : Widget). The Parent : Child format should not be used under this column.
We can also use the Internal Id of the Child Matrix Item instead.
Navigate to Home > Set Preferences > General tab > Defaults section and set Show Internal Ids to True to show the internal id on a list.