This line if reasoning is such bullshit, sorry. The underlying tone is always "childfree people are selfish", with the underlying message "parents are such giving people", as you pretty much explicitly say in your comment.

Except I have known plenty of parents, including some with gigantic families, who are some of the most selfish, narcissistic people I've ever met. They basically saw their children as an extension of themselves, like they were building some "noble family" bullshit or something. Simultaneously, I've also seen people without children who have dedicated their entire lives to public service and helping others.

Of course not all parents and people without parents are like this. I personally have never seen any evidence that those with or without kids (voluntarily) differ in their giving nature towards others.