Child records are not editable. Best choice would be to set a row action to popup or drawer with the child records in its model.

Only fields on the model’s chosen object can be edited in that context. To accomplish what you’re after, what I’ll often see is a secondary model (or more than one) built on the related objects you’re trying to make editable. That secondary model can take its record context from any given row on the table, and can be shown inside a drawer or popup using a row action. The context would need to be set so that the record that was clicked on has its Id passed into the secondary models as a condition. 

Since the drawer’s component(s) are connected to the secondary model(s), they can be editable as needed. 

This tutorial does a good job of illustrating the arrangement I’m trying to describe. I hope it gives you some ideas about how to approach your use case. 

Thanks Mark.  That is the solution I am currently employing.  I was hoping I just missed something.