I was thinking that maybe it has to do with the actual features of the children, in the case of biological kids; it makes sense; we are familiar with our own facial features and the facial features of the one we married. And children's features are exaggerated by Mother Nature to make them cuter-- huge eyes in relation to the face, round noses, round faces, plumpy cheeks. So it's like a cute-i-fied version of ourselves and/or our lovers.
In the case of adopted babies, I think the same adoration comes from 1) the Mother Nature exaggerated cuteness of all babies (think of Disney exaggerated eyes and round noses) and also 2) from knowledge and familiarity-- the recognition of what that look means, what that blink means, what that gesture developed from.
I love babies so much. Even when my baby is screaming his head off, I just want to kiss his sweet baby lips and cheeks and his cute little neck and his sweet little shoulders and bum. Babies are just the best!!!