The only place you get significantly more than you put in is Medicare, and that's because healthcare costs in America are out of control. More kids doesn't help that.
[1] https://www.forbes.com/sites/chrisconover/2012/12/03/aarp-lo...
That's because it is an "I'm paying for my parents" fund. When the system started out it paid benefits to people who never paid into the system. The only reason there is any money in the "trust fund" at all is a result of population/GDP growth. If there was a population decline from too many couples having only one child it would bankrupt the system.
Moreover, the social security "trust fund" consists of government bonds, but bonds held by the government itself are just self-canceling worthless paper. Every dollar of credit is also a dollar of debt that the people who supposedly paid into the system actually spent on past government services. It's their children who inherit that debt whether you call it a "trust fund" or not. And if they have no children, the debt falls on someone else's children.
> Remember, too, that if you're self-employed or make a lot of money you're going to get back significantly less than you put in because of the way the caps work.
There are corresponding caps on social security tax for the same people. And self-employed people don't actually pay more, the "employer portion" is a subterfuge to make the social security tax rate seem to be half what it really is. Employers just deduct it from what they're willing to offer as compensation, which makes it seem like people are being paid less when they're really being taxed more.