I'm not sure about its actual origins, but that slogan was introducd by David Bergland when he ran as the LP Presidential candidate. As I recall him explaining it, infants are almost absolute Libertarians, then as they get programed first by their parents, then by society, they become less and less libertarian. The examples he used were very one sided and positive. It sounded good, on a quick hearing, but didn't hold up on further examination.
Individual rights is a two sided idea. As the LP's principle of living your life as you choose, also holds the proviso of not interfering with others to live their lives as they choose. Infants, babies, small children, have no respect for other peoples rights, and thereby cannot be libertarian.