This episode features Peter Capaldi, who later becomes the 12th Doctor in Doctor Who (2005).
Having been set up to do so by the conclusion of her storyline in Journey's End (2008), Freema Agyeman was due to reprise her role as Martha Jones. However, she was unable to due to "scheduling issues". Russell T. Davies explained that Agyeman was cast in Law & Order: UK (2009) before Children of Earth had been officially commissioned. Because Law & Order offered her 13 episodes a year, she went with that over Torchwood, which had been reduced to five. In response, Davies created the character of Lois Habiba, played by Cush Jumbo, to be a "kind of a Martha figure", one with added innocence who is out of her depth.
Fliming took place on the set of BBC's Casualty (1986) in Bristol, which doubled for the fictional St. Helen's Hospital in Cardiff.
Rupesh Patanjali says that he is originally from Chesterfield, England. Rik Makarem actually is from Chesterfield.
Gwen disparagingly calls Jack and Ianto "the Chuckle brothers". This is a BBC children's slapstick comedy for which writer Russell T. Davies wrote three episodes early in his career.