Utter BS, there is no political correctness in thinking about the morality in the fantasy world you are playing a RPG in. In fact it's one of the interesting things about it. If we wanted to play just killand who cares there are plenty of board games that would do a better job. Why does one character kill the goblin women and children and another one doesn't? Because moral questions are interesting drama 101. It's not just a game, it's a game where you make up a story that's worth taking part in.
In this case it's an interesting question, some characters might want to try to capture and domesticate a young monster or just leave them alone as part of the natural world. Others might not. Being able to role-play both convincingly is a sign that you are winning D&D I suppose.
If you want to play the game as just kill everyone you meet for XP and take their stuff and your group agrees that's great but making a stink at other people for doing it differently is a dick move.

There are two issues here.
One issue is - is it "all right" to kill a child if that child is a threat to your life?
The second issue is - should a publishing company be creating fictional monsters for a game that are dangerous children to be slain?
These issues are very separate.
Pretty sure that baby murder is not a "21st century liberal" thing.
D&D is also not a medieval Earth game. It features magical robots, air ships, lizards that are smarter than humans, and angels and demons you can play poker with.
I agree that it is a bit myopic to make monstrous humanoid infants a monster type, but in all fairness, 5E is aiming for the old days, which were built heavily on fantasy racism and little thought put into what being a murder hobo really means from an ethical standpoint.
As a DM, I might just use these creatures specifically to put some massive guilt down on the PCs, and to basically put them in the shoes of being the real monsters.
Overall, I agree that WotC really should keep the problematic nature of these sorts of depictions in mind, and to avoid having infant forms of intelligent monsters in this way. It may not be the most vile of acts to create such a bit of fiction, but there are many other ideas available which are less prone to problems.
Worth keeping in mind that "it's a game" isn't a defense any more than "it's a book" or "it's a conversation." Games are expressions of ideas, along with their mechanics. One does not automatically agree with the implications of their art (good people can write stories about evil people!), but it does still express ideas, and offers them up for criticism.
Why does there need to be a defense? There is nothing wrong morally with killing fictional infant creatures.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.