(which is, of course, because it wouldn't be practical or humane to subject a live infant to an arduous prosthetic makeup process.
Well...they DID on the borg baby...
Not really. The Borg infant in "Q Who"
just had a sort of "bonnet" slipped on his head and a box on his chest -- more costuming than prosthetics, nothing that would be difficult to apply and uncomfortable to wear the way, say, a full Jem'Hadar makeup would be. And the Borg infant from "Collective"
was a puppet.