From the advertising including booklets issued by "The Lydia Pinkham Medicine Co." I doubt that the baby in every bottle was ever used by the company in its claims (usually stressing that 98 of 100 women receive benefit from using the Compound but not promising specific cures). It may have appeared in one of the many endorsements of the products which were sent in (?) by users and quoted in booklets and advertisements.
The endorsement letters are complete with names and street addresses and hundreds were printed. See American Memory (search Lydia Pinkham) for examples. Genuine or faked? I haven't read (and doubt that I will) any of the scholarly papers on the Compound and other popular remedies of the time.