In the original unaltered script by Brendan Hood, the plot dealt with a group of four recent college grads who discover that Earth is actually run by a race of organic machines which allow the human race to exist so that they can kill them and harvest their corpses for 'spare parts' as "Their" bodies break down and deteriorate.
Contrary to the marketing's claims, Wes Craven had no involvement in the production of the film. The complete U.S. title for They is "Wes Craven Presents: They." Craven was considered an 'executive producer' of the movie. However, other than lending his name to the title, he had no part in the making of They (2002). It's assumed that the purpose for putting his name in the title was to publicize the movie and attract horror fans.
Though Brendan Hood is credited as screenwriter, he only came up with the basic idea. Everything in his original draft was subsequently written out by the producers, and it's estimated that up to 10 people have worked on the script. Only one of them was credited for the revised script.
The distribution rights to the film were bought by Dimension Films upon seeing test footage of this film in England.
The explanation for why "They" target certain children and return later to hunt them down is ambiguous. One theory is that "They" are doing what human researchers do when we study a species which is to tag the specimen, release it, and later use the tracking device to find it again in order to chart its progress and development. Also similar to human researchers, "They" won't tag and study every single creature in the breed, just a random few to study in depth.