Uh, the link above does not go to the official LAX website, but to "ifly.com" which appears to be a site with lots of information on different airports (as well as paid advertising for the travel industry) but it's not clear where all this info comes from or if it's all correct. The "official" LAX website is
LAX and just sends you to the TSA website for questions about security screening, identification, etc.
Look, I'm not trying to be a pain or argumentive here, just trying to get correct info out and determine if something has changed in this regard recently (which I don't think it has). From your experience, it would seem that Allegiant may be the exception to children maybe needing ID altho we don't know why the airline wanted the birth certs (maybe the reservations didn't indicate that they were kids?) The Alligiant website doesn't have any info about ID requirements so it's not clear what their policies are. But we do know from the websites of most major airlines that id is NOT required of children generally. So if JetBlue says it's not needed, you have your answer

. And if you're in doubt check with your carrier...
Happy trails!