Does is_child() exist in wp 3.5.1?

To check for a specific parent ID use $post->post_parent=="123" and replace “123” with the parent ID of your choice.

I learned how to do this by using the Widget Logic plugin. From the Widget Logic notes:

global $post; return (is_page('pagename') || ($post->post_parent=="13")); — home page OR the page that’s a child of page 13

Edit: A better source is Testing for sub-Pages in the codex.