Hey,
(Created from Execution Public API )
Currently, there is no way to structure the execution list. If you have a workflow x, which calls a subworkflow y, there will be 2 executions in the log, but they are not linked (like retry).
There is no way to build a tree-structure monitoring for executions via public api 
It seems there is already code for it, but not fully implemented ( parentExecutionId ):
executeWorkflow()
workflowInfo: IExecuteWorkflowInfo,
additionalData: IWorkflowExecuteAdditionalData,
inputData?: INodeExecutionData[],
parentExecutionId?: string,
loadedWorkflowData?: IWorkflowBase,
loadedRunData?: IWorkflowExecutionDataProcess,
NodeExecuter (only 3 params…)
.executeWorkflow(workflowInfo, additionalData, inputData)
Guess it will be very nice to link both of the workflows. Specially, when you call a sub sub sub sub sub workflow.
An ugly alternative way would be calling an own implementation service after every subworkflow to persist this data. Sub Workflow would return item.executionId = parseInt($executionId.toString()) and the main workflow would call the service. But this seems like a really terrible idea 
Best,
Rf.