There are six child characters in the show: Himari, Shouma, Ringo, Kanba, Masako, Mario, and two main families, the Natsumes and Takakuras, each of which are high-level executives in the Penguin Force/KIGA group. Penguin Force/KIGA may actually be two separate groups each run by one of the families, with the same ultimate goal but different methods. Not sure, still trying to figure it out, will probably become much more clear as the show ends.
As far as familial relationships go... every one of the six children is a member of the natsume or takakura family either biologically or through adoption (excluding Ringo).
Ringo: Not a takakura or Natsume in any way, shape, or form. (But she may end up hitched to Shouma, making her a takakura
)
Masako: Biologically and raised as a Natsume. The younger sister of Kanba biologically.
Shouma: Biologically and raised as a takakura.
Himari: Bioologically, she's not a member of either family. She's was just a random neighbor of Shouma. "Adopted" by Shouma as a Takakura.
Mario: Raised as a Natsume. Biological origin/relations not completely certain yet I think.
Kanba: Biologically a Natsume, and Masako's older brother. Somehow he ended up being adopted as a Takakura. Perhaps he was banished, seen as unfit to lead the Natsume family by their grandfather?
Both Shouma and Kanba seem to have had their own fate apples.