Whatever is attractive about a human infant is not any of the usual attributes of esthetics. The truth is that infants are ugly, misshapen, and asleep (except when hungry or wet). It is a quirk of perception that they can (and are) looked upon as if they were cute and adorable.
Well, I suppose that infants are "adorable" in a literal, etymological sense. They are in fact adored and so must be capable of being adored.
It would be more useful to say that the infant is lovable. What is important is that larger humans behave as if the infant human is attractive in the ordinary senses of that word. We can only hope that close relatives of newborns don't read this musing for several years, because they are convinced that their baby meets all the criteria for visual beauty (and may claim olfactory beauty as well).
A potential for some sort of incipient comeliness may be visible even in the youngest children. I'm quite certain that's not what the adults find attractive about them, however. What attracts the adults is that the baby is misshapen and helpless in specific, stereotypical ways which record as infant human in the adult brain.
That category of infant human engenders responses such as closeness, protectiveness, and various other kinds of service which are useful to the infant (and the species). These responses are generally rather costly to the adult, whose only immediate benefit is emotional. A reductionist view would argue that there is nothing else. The fact is that we can plausibly (if speculatively) explain parental activity entirely on the basis of innate infant recognition cues and responses.
One needn't be so completely reductive, though, to recognize that the attraction of infants is a distinct class of human response. To the parent or caring adult, the appearance of the infant in their sight feels like the apparition of pure beauty. But it isn't.
The actual reality is stronger and deeper than pure beauty, more profound than cuteness.