There was a lengthy discussion here (n.b., I'm not joking when I say "lenghty", please be careful if attempting to read it all) where some editors involved agreed that citing the film as a primary source is acceptable and others did not. There is a template (Template:Cite video) that can be used to cite a video available though a URL but, obviously, you'd have to be sure that the URL steers clear of Wikipedia:Copyright violations. Personally, I would suggest not to use the film as a source for this sort of thing because secondary sources are always better, if available. Also, you want to avoid violating WP:SYNTH which is done when you present a conclusion that you reached yourself by citing a primary source. But, if you do decide to cite the film as a source, you can use reference #68 from this version of Children of Men as a guide of how to format a citation to the actual film.  (talk • contribs)