Programatically add to child table in webform

In desk forms there’s frm.add_child, is there an equivalent function in web forms? Or has anyone have some set of codes that would insert values in the child table in a webform?

Use case is I’m trying to add a feature to copy some data from a source DocType to another in the webform and the source DocType has a child table fields. I’m able to copy the other normal non-child table fields.