Show same records from one child table under one doctype to same child table under another doctype

I have one child Table called Sales Variant. I put that table under Item Group DocType. Now If Item Group has one Finished Goods with single or multiple records in that child Table Sales Variant. I want to show that Sales Variant records in Item record if Item Group is Finished Goods.
Below you can check code I written as screenshot I have attached for better understanding. In Item DocType Record is visible but If I try to save Item Record it’s creating record in Sales Variant as taking Item as parent.

    // As of now Table should be readonly
    frm.set_df_property("sales_variants", "read_only", 1);

    if (frm.doc.selling == 1) {
        frappe.db.get_list("Sales Variants", {
            filters: {'parentfield': 'sales_variants','parenttype': 'Item Group', 'parent': frm.doc.item_group},
            fields: ["*"],
            limit: 20
        }).then((res) => {
            frm.doc.sales_variants = []
            $.each(res, function(_i, e){
                let entry = frm.add_child("sales_variants");
//         =;    // If I try to store name field record is not showing as expected. 
                entry.item_code = e.item_code;
                entry.item_name = e.item_name;

Why not use item variants?

At first glance, it seems like you’re trying to achieve the same thing in a different way.

Thanks for your time and reply @rmeyer

I know that Erpnext has default functionality but the requirements which I have need to create that customise functionality. As I need that in Item DocType, In Sale Order Item has the same variants which I have showed under the same Item DocType Variants.