Selfishness is commonly understood as putting one's own interest above everyone else's to an excessive degree, not the interest of their family. If you redefine it to include family interests then I think everyone would agree that having a family makes people more selfish, often dramatically so.

I just object to the redefinition.

I think this "redefinition" is apt in the context of the conversation. It's about people mistrusting those without kids because they are viewed as selfish, and I'm pointing out that they're often the least selfish, and more likely to "help the world" in situations where they do not benefit.