Instead of having biological children of his own, Seto Kaiba adopts one. Mokuba does the same thing, except he adopts more. Seto, and maybe also Mokuba, do not have the time to constantly care for the children so the children are constantly under care of domestic workers. The reason Seto adopts one despite his lack of time is because he wants someone he raised to take over the company after he and Mokuba are gone. The children that Mokuba raise are trained to work as executives for Seto’s child. Seto also wants some time to spend with the child and to raise them to the best of his abilities. While he could have a wife to take care of the child, he is not interested in finding one. He would rather be concerned about himself, the company, Mokuba, and the child.
I can actually see this happening. Seto has a lot of compassion for children, especially orphans because him and Mokuba were one and he knows how it feels like. With Mokuba though, I see him having his own children, and then maybe adopting one. Seto would adopt two, who happen to be siblings. I’m not sure about the kids being raised to be heirs or executives though. I personally don’t think Seto would want his kids to live in the same strict and abusive lifestyle he did, so he’d give them more freedom.