Updating child row does not lock parent row in SQL Server

child_parent_update_locking_demo.sql

-- Tested on SQL Server 2008 R2
-- Isolation level is READ COMMITTED

--
-- Session 1
--
CREATE TABLE dbo.parent (
	  parent_id	INT				NOT NULL	PRIMARY KEY
	, value		VARCHAR(50)		NOT NULL
);

CREATE TABLE dbo.child (
	  child_id	INT			NOT NULL	PRIMARY KEY
	, parent_id	INT			NOT NULL
	, value		VARCHAR(50)	NOT NULL
	
	, CONSTRAINT FK_child_parent_id	FOREIGN KEY (parent_id) REFERENCES dbo.parent(parent_id)
);

INSERT INTO dbo.parent 
	  (parent_id, value)
VALUES 
	  (1, 'howdy')
	, (2, 'check it out');
;

INSERT INTO dbo.child 
	  (child_id, parent_id, value)
VALUES
	  (1, 1, 'there')
	, (2, 1, 'mang')
	, (3, 2, 'dude')
;


BEGIN TRANSACTION;

UPDATE dbo.child
SET value = 'over there'
WHERE child_id = 1;

--ROLLBACK TRANSACTION;

--DROP TABLE dbo.child;
--DROP TABLE dbo.parent;



--
-- Session 2
--
UPDATE dbo.parent
SET value = 'hey you'
WHERE parent_id = 1;