diff --git a/tests/phpunit/tests/post/getPageChildren.php b/tests/phpunit/tests/post/getPageChildren.php
new file mode 100644
index 0000000000..356fa271b4
--- /dev/null
+++ b/tests/phpunit/tests/post/getPageChildren.php
@@ -0,0 +1,95 @@
+<?php
+
+/**
+ * @group post
+ */
+class Tests_Post_GetPageChildren extends WP_UnitTestCase {
+	protected $pages = array();
+
+	/*
+	 * Here's the tree we are testing (4 is not in the tree):
+	 *
+	 * pages[0]
+	 * - pages[1]
+	 * -- pages[3]
+	 * - pages[2]
+	 * -- pages[6]
+	 * --- pages[7]
+	 * ---- pages[8]
+	 * - pages[5]
+	 */
+	public function setUp() {
+		parent::setUp();
+
+		// Mock page objects.
+		$this->pages = array(
+			0 => (object) array(
+				'ID' => 100,
+				'post_parent' => 0,
+			),
+			1 => (object) array(
+				'ID' => 101,
+				'post_parent' => 100,
+			),
+			2 => (object) array(
+				'ID' => 102,
+				'post_parent' => 100,
+			),
+			3 => (object) array(
+				'ID' => 103,
+				'post_parent' => 101,
+			),
+
+			// Not in the tree.
+			4 => (object) array(
+				'ID' => 104,
+				'post_parent' => 9898989898,
+			),
+
+			5 => (object) array(
+				'ID' => 105,
+				'post_parent' => 100,
+			),
+			6 => (object) array(
+				'ID' => 106,
+				'post_parent' => 102,
+			),
+			7 => (object) array(
+				'ID' => 107,
+				'post_parent' => 106,
+			),
+			8 => (object) array(
+				'ID' => 108,
+				'post_parent' => 107,
+			),
+		);
+	}
+
+	public function test_page_id_0_should_return_all_pages_in_tree_and_exclude_pages_not_in_tree() {
+		$expected = array( 100, 101, 102, 103, 105, 106, 107, 108 );
+		$actual = get_page_children( 0, $this->pages );
+		$this->assertEqualSets( $expected, wp_list_pluck( $actual, 'ID' ) );
+	}
+
+	public function test_hierarchical_order_should_be_respected_in_results() {
+		$expected = array( 100, 101, 103, 102, 106, 107, 108, 105 );
+		$actual = get_page_children( 0, $this->pages );
+		$this->assertEquals( $expected, wp_list_pluck( $actual, 'ID' ) );
+	}
+
+	public function test_not_all_pages_should_be_returned_when_page_id_is_in_the_middle_of_the_tree() {
+		$expected = array( 106, 107, 108 );
+		$actual = get_page_children( 102, $this->pages );
+		$this->assertEquals( $expected, wp_list_pluck( $actual, 'ID' ) );
+	}
+
+	public function test_page_id_that_is_a_leaf_should_return_empty_array() {
+		$actual = get_page_children( 103, $this->pages );
+		$this->assertEquals( array(), $actual );
+	}
+
+	public function test_nonzero_page_id_not_matching_any_actual_post_id_should_return_empty_array() {
+		$actual = get_page_children( 200, $this->pages );
+		$this->assertEquals( array(), $actual );
+	}
+}