A shadow child was a son or daughter born to an important Chiss family, whose birth and identity were kept secret to prevent the entire bloodline being exterminated by rivals.
Baron Soontir Fel and his wife Syal Antilles Fel were said to have had five children, but evidence existed for a total of six: four sons, Chak, Davin, Jagged, and Cem, and two daughters, Cherith and Wynssa. Cem was revealed to be a shadow child of the Fel family.
Behind the scenes
The only recorded allusion to the custom was made in order to explain an apparent paradox in the Fel family. The introduction of the character Chak Fel in Survivor's Quest in 2004 created a continuity error by adding a sixth Fel child when five Fel children — five having been established as the total number of Fel children — had already been identified. As Davin Fel was the only Fel child of similar age to Chak, Lucas Licensing editor Sue Rostoni initially suggested that Chak was in fact Davin, with Chak being a nickname. This suggestion was ultimately not integrated into canon, with Dark Nest I: The Joiner King introducing the "shadow child" custom as an explanation.
While it may be that "shadow children" were a true Chiss custom, it is also possible that the explanation was supplied as misinformation to disguise the true reason for this discrepancy.