Well, the direct answer is: these are all Americans, and all pay some kind of tax to their city, county, state, and ultimately the country, in theory so they can draw on the shared resources of the nation. That's how you justify supporting the kid in Mississippi and not the kid in Cambodia.
As for rationally holding the belief that people can have as many kids as they want, and those kids will be cared for, yes, you can hold those simultaneously. I don't want any kids in my country to go hungry; how many siblings they have or what choices their parents have made are irrelevant to those individual kids' needs being met. There's no conflict there.
We live, as they say, in a society.