Step Functions: Step Functions StartExecution Exception

I am trying to prototype a pipeline that uses nested workflows in order to build a data processing pipeline. My POC is very very basic.

1. Parent workflow has one state, call the child workflow
2. The child/nested workflow invokes a lambda.

The above was created via the web console to get things working before moving to SAM. The Child Workflow in (2) above works when starting the execution manually via the console.

However, when attempting to start the execution of the Parent Step Function manually, I can't get past the following error:

I added the Step Functions Full Access to the IAM role used by the parent Step Functions but that did not resolve the issue.

What do I need to set in order to allow a "parent" Step Function to call and leverage nested/child step functions. In the future I plan on doing more advanced operations like Map, etc., so I am simply trying to sort out the core bits needed via this painfully simple POC building out my solution.

Thanks in advance.