You could, but there is also a "Start Workflow" action. Why not use that?

I assume new item is created in that child list.

then configure workflow on child list to run automatically on item creation.

once the master workflow creates a new item, workflow will automatically be fired by sharepoint.

Hi , 

Thanks for your response but I have already configure my child workflow as per your suggested options but it is still not working. Maybe  Nintex for Office 365 doesn't support this functionality.  

I'd say it should start just the same way.

does it start if you create child item  manually?

Yes!!

sadly, it seems this is really not implemented on O365

Office 365 Workflow Not Starting Automatically 

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