How to add a new child row to an existing parent record

Frappe Framework
1

How to add a new child row to an existing parent record.

Parent Row
admission No : 1001
Student Name : Alex
Class : 7 A

Child Row
Subject : Maths
Marks : 45
Subject : English
Marks : 30

Need to add a new row without parent

Subject : Social
Marks : 48