what is the parent child relationship difference between process builder and workflow

Hello Raghav,
You can use the Process Builder to perform more actions than with workflow:

Create a record
Update any related record
Use a quick action to create a record, update a record, or log a call
Launch a flow
Send an email
Post to Chatter
Submit for approval
Call apex methods

But the process builder doesn’t support outbound messages.

Workflow does only 4 actions

Create Task
Update Fiel
Email Alert
Outbound Message

More Info:

https://help.salesforce.com/articleView?id=process_overview.htm&type=0