Origin of "Rock-A-Bye Baby"

I searched the archives, but either I was doing it wrong, or there really isn’t anything there about this (my guess is that I’m doing it wrong), but I’m curious as to the origin of this nursery rhyme, and it’s meaning.

It’s one of my three-year-old’s favorites, but I just don’t get it. Why is the baby in a cradle in a tree? And why don’t children completely freak at the thought of that baby falling out of the tree when the rhyme gets to the “when the bough breaks” part? It’s not “if” the bough breaks, which would imply that perhaps the baby could stay up there safely for a while. Noooooo. It’s “when,” which states quite plainly that that kid is definitely going to come crashing down, eventually becoming a tangle of broken bones and smashed cradle.

Can anyone help me out here?

I looked it up on Lycos and this is what I found

http://www.sca.org.au/bacchus_wood/origins_of_nursery_rhymes.html

It says that a young pilgrim boy observed American Indians building cradles for thier babies out of birch limbs and hanging them in the trees for the wind to gently rock them to sleep.