To add a movie/TV Show to your Kids Queue, go to your "My Queue" page, click the icon on the top left of the movie/TV Show you wish to move and then select "Manage" then "Move to Kids Queue".
Updated
To add a movie/TV Show to your Kids Queue, go to your "My Queue" page, click the icon on the top left of the movie/TV Show you wish to move and then select "Manage" then "Move to Kids Queue".