Hi Abhishek,
You can create call back event function module and register it on save of the document.
For registering the EC function module in spro you can find many docs in scn forum.
You can read the docflow table using crm_order_read_ow(fm) by passing the parent document guid then identify the follow up documents.
You need to set the custom field value in sub documents using crm_order_maintain function module.
Best Regards,
Dharmakasi.