It honestly never occurred to me that we would need it - yes, I know the USVI is a territory, but as I said we were visiting family who live there - we knew we wouldn't need passports, so it seemed logical to me that if we didn't need passports, we wouldn't need her birth certificate. The very first time we flew with her when she was tiny, we did bring it with us - and we did leave the lower 48- we went to HI. And no one asked us for any documentation for her at all. All they did was ask her name, and give us her BP.
And I was just pointing out that, heaven forbid something happened to my own child, and were I the sort of person who would kidnap another child, simply showing a piece of paper with my name and a name of a child who is as yet unable to talk to identify herself, is hardly the perfect security check that the officer was making it out to be.