> It’s really because statistically, in my experience people without kids are more selfish than those without.

I've witnessed the opposite: Having kids made people much more selfish. Resources were plenty before they had kids, so they would spend a lot (time and money) on others - be it friends or the general public.

When kids come along, two things happen:

1. Resources are limited, so a lot less goes outside the family.

2. At least one parent will put the foot down when being generous to people outside the family - even if the wealth/income supports being able to do so. Tribalism sets in.