What about them? Those kids are themselves either citizens or (improbably) legal permanent residents (and thus, beneficiaries) or not (and thus not). This isn't rocket surgery.

> Will people in retirement continue to get their social security?

That depends whether Social Security is a replaced program or not (IMO, it shouldn't be, since its not a means-tested social benefit program.)

> How will we pay for all this?

With new taxes plus the redirected funds from replaced programs.

> Assuming 418M people

From a few different sources, I get that the US population is 319M, about 93% citizens and 3.6% lawful permanent residents. So, assuming 418M beneficiaries of a UBI is unreasonable.

> at $13K which I typically see mentioned as basic income

$13K/capita is a just about the point at which two-person household would clear the poverty line, so its not a bad point to consider a reasonably mature UBI; OTOH, I'd prefer to identify a funding mechanism that should grow with economic growth, and have benefits scaled so as to (in the long-term, smoothing short-term variability) follow that funding mechanism, eventually reaching such a mature level (and going to even higher levels with more growth) rather than setting a benefit level and then trying to fund it out of the gate.

And I wouldn't expect a mature UBI for a generation or so.

This is not the slippery slope that you're implying; you can certainly draw certain lines when the system that you're building has limited scope and leaves the outliers to other benefits systems.