You can include child work items while copying a work item and creating a new instance from it. Copy child work items could be very useful when you want to create a new work item from existing items that have several other work items associated.
Copy Child Work Items in Azure DevOps
To copy a work item, from the work item, select “Create a copy of work item” option
Related Tip: Link Azure DevOps work items to an existing build
While copying a work item, choose “Include child work items” to the copy associated work items.
This option will copy the work item and copy all child work items (up to 100).
Want to develop custom solution using Azure DevOps – Check this out : Exploring Azure DevOps APIs