I can suggest two things here.
1st- You can define an out_Error argument in the beginning of each flow, assign it to false. If there is some error, you can catch it in try catch block, set out_Error argument to True in the catch block.
And after this sequence, you can check condition in the parent flowchart that if out_Error argument is False, proceed to the next step, otherwise, send an error mail notification. (You can create this send error mail notification workflow and reuse it in subsequent checks).
2nd- You can use Try catch in the child workflow and in case of any errors, you can use a throw activity with the exception message so that it gets caught in the parent workflow. Inside catch block of parent workflow, you can use Send email (with exception.Message in Body) to notify the concerned people.