The Interspecies Family is referred to once in the book, while Wanda, Ian and Jared are on a raid. They go to a motel and see the family from the window. Wanda referres to the family as 'hope'.
The family consists of a soul father, a soul mother and a baby who is very much human. The family is said to be one of a kind. Wanda believes that the Seekers would not force a soul mother to give her human baby as a host body for a soul. All of the humans and Wanda are transfixed by this concept, and watch the family.
While being watched, the soul father is pushing the human baby on a swing while the soul mother is catching him.