Okay this is bizarre o.o
I just kept breeding Emmas, and I just got the achievement. This kid is the same as all the other Emma creatures genetically. From his family tree, I got him by breeding two Emma creatures together, but besides his parents, he also has aunts, uncles, and grandparents who fit the requirements and didn't trigger the achievement. Even weirder is that some of them just were born just a few minutes ago. Is it both parents also fitting the requirements? I'm not sure what happened, but at least it worked eventually??