When I first read 5HC, I imagined the shadow children looked something like the orcs from Ralph Bakshi's Lord of the Rings movie. Maybe it's the way the rotoscoping in that film made the orcs seem somehow separated from their environment and the other characters, a separation that their odd color palette accentuated. Anyhow, that weird quality fit perfectly with my picture of the surreal, bestial shadow children.
Also, Miyazaki's apes from Princess Mononoke are a pretty good match.
How do you guys picture the shadow children?