In article ,
Judah wrote:

I was under the impression it was 2 and under.

It's "until the child has reached his second birthday." It's still a
monumentally stupid idea to try and hold a kid that young, but it's
legal for the moment. In severe turbulence or an accident, it will be
*impossible* to hold onto him without killing him.

The point being, what
documentation does a parent need to provide to prove the child is within the
age limit?

Many airlines require a birth certificate be presented to prove the
child is under 2. For example...

http://www.southwest.com/travel_center/infants.html
"A birth certificate is required to validate age of all infants under
age two."

Presumably, the parent simply says the child is under 2, and the problem goes
away.

Not likely.