I've been traveling by air with three children for 11 years. All my kids have their own frequent flyer accounts. After I flew with my oldest at 9 months, I swore I'd never do a lap baby again. We've bought tickets for them, rather than going lap-child, even when they were young enough to save the money. They definitely ask the child's name. They write it in the flight manifesto. They also put a big neon sticker on my boarding pass saying "infant" and they add the child's name there.
When we go through security now, the TSA officers look at the boarding passes, and then they look at the five of us. They say, "who is Bobby" and they look at the kids to see if they'll answer for themselves. They have never asked for ID from the kids unless its international. But you wouldn't expect an infant to answer.
My 11 YO flew alone for the first time last summer. in that case he had to have photo ID, even for a domestic flight. We have traveled internationally so, he used his passport.
I don't think it would be that difficult to get on a plane with an infant w/o ID. with the Amber alert issued though, I'll see what I can find about airports & amber alerts. I bet they get the notice. I'd wonder too, since I live in Texas, surely Border patrol gets those Amber Alerts too? They must, right? I hope so!