Overall, it's better for the game and the playerbase if they keep reduced-faculty infant forms of intelligent creatures out of the official rules. Such topics are best left to mature-oriented fiction rather than fiction for children on up. You can still have intelligent infant creatures or infant unintelligent creatures without much potential for conflict, as the former is capable of moral/ethical choices and the latter has no later capacity for such. Helpless semi-intelligent infant creatures, such as real world infant mammals, are also best avoided; there just isn't anything gained by enabling lion cub smiting.
In a horror genre game oriented toward adults, such as the Alien franchise, it's perfectly normal to have more monstrous infants with a future of intelligence, as this genre is often oriented around people doing horrible things and suffering evil because of their "sins."
Within the context of a campaign, players defending themselves against dangerous infants of any sort can be perfectly reasonable, whether or not the implications of the action are explored, but there really isn't a value to devoting design space to this. There is less of an issue of suggesting that something MIGHT be the infant form of something that will develop intelligence, as this allows the DM to make decisions based on their audience or personal preference without having to actually change the established fiction of the game.
I personally use a lot of horror in my games, including infant forms of intelligent creatures suffering, and variously tempting my players to play by the rules of great evils, but this is an active choice on my part, and isn't driven by the game's own suggestions. I wouldn't mind a less gross-out-oriented book of vile monsters which are only appropriate to games with adults or adult approval, but the base game doesn't need to spark deeper moral questions than whether to side with the dwarves or the elves in a resource war.
Moreover, whether or not an individual appreciates it, numerous sci-fi and fantasy fans (including more mainstream fans) get uncomfortable around depictions of intelligent beings being dismissed as moral entities simply because of their form. Large amounts of popular fiction are built around the issue, including Dragonheart, Avatar, and Hellboy, and increasingly the issue comes up as a side plot in many broader works. Given the nature of D&D as a franchise, its better to be inclusive of that section of the audience, and to minimize the chafing between them and those with more comfort with the literal monstrous Other by simply not making it a point in the fiction.
