API 'EGO_ITEM_PUB.Assign_Item_To_Org' can be used to Assign Item to Child Org.

Below code helps Us to Understand more on the API

declare
l_mesg VARCHAR2(1000);
l_count number;

begin

--Getting the Organization id
BEGIN
SELECT Organization_id,master_organization_id
INTO v_organization_id,v_master_org
FROM mtl_parameters mp
WHERE mp.organization_code = 'C1'; --C1 is the Child Organization Code
EXCEPTION
WHEN OTHERS THEN
dbms_output.put_line('Error in getting the Organization id for Organization code V1 and error is '||SUBSTR(SQLERRM,1,200));
END;

--Getting the Inventory Item id of the Item which is available in Master Organization
SELECT inventory_item_id
INTO v_item_id
from mtl_system_items_b
where segment1 = 'Existing Item Name'
and organization_id = v_master_org;
EXCEPTION
WHEN OTHERS THEN
dbms_output.put_line('Error in getting the inventory item id for Item and error is '||SUBSTR(SQLERRM,1,200));
END;

EGO_ITEM_PUB.Assign_Item_To_Org(
p_api_version => 1.0 ,
,p_Item_Number => v_item_id
,p_Organization_Id => v_organization_id
,x_return_status => l_mesg
,x_msg_count => l_count);

Commit;

IF l_count =1 THEN
dbms_output.put_line( 'Error in API is '|| l_mesg);
ELSIF l_count >1 THEN
LOOP
l_count := l_count+1;
l_mesg := FND_MSG_PUB.Get(FND_MSG_PUB.G_NEXT,FND_API.G_FALSE);
if l_mesg is NULL Then
EXIT;
END IF;
dbms_output.put_line('Message' || l_count ||' ---'||l_mesg);
END LOOP;
END IF;

Exception
dbms_output.put_line('Error in the API and error is '||SUBSTR(SQLERRM,1,200));
END;