Are babies of evil humanoid species inherently evil?

tl;dr: With some exceptions (such as fiends, which are innately evil), alignment is a description of the moral outlook and attitude towards society of an Individual. Racial tendencies are not hard and fast rules that apply to every member of the species. Creatures incapable of rational thought (including the baby in your example) cannot make moral choices and so are desalinhado.

Vamos dar uma olhada as regras:

The first interesting sections are these:

A typical creature in the game world has an alignment, which broadly describes its moral and personal attitudes. Alignment is a combination of two factors: one identifies morality (good, evil, or neutral), and the other describes attitudes toward society and order (lawful, chaotic, or neutral).

e

For many thinking creatures, alignment is a moral choice. Humans, dwarves, elves, and other humanoid races can choose whether to follow the paths of good or evil, law or chaos. According to myth, the good-aligned gods who created these races gave them free will to choose their moral paths, knowing that good without free will is slavery.

e

Most creatures that lack the capacity for rational thought do not have alignments—they are unaligned.

So, morality is a escolha, and a baby lacks the capacity for rational thought so a Duergar baby is desalinhado.

For the sake of completeness:

The evil deities who created other races, though, made those races to serve them. Those races have strong inborn tendencies that match the nature of their gods. Most orcs share the violent, savage nature of the orc gods, and are thus inclined toward evil. Even if an orc chooses a good alignment, it struggles against its innate tendencies for its entire life.

I don't think a tendency towards evil would override the inability to choose.

Alignment is an essential part of the nature of celestials and fiends. A devil does not choose to be lawful evil, and it doesn’t tend toward lawful evil, but rather it is lawful evil in its essence. If it somehow ceased to be lawful evil, it would cease to be a devil.

Duergars are not fiends, so alignment is not an essential part of their nature.

To add to what was already said, for higher thought creatures, we can observe the tieflings.

In XGE, P.62, the tiefling heritage states that one parent was a tiefling or human and the other could be either a human, tiefling or devil. In that sense, regardless of how they came to be, one thing is clear. Infernal nature courses through their veins.

Despite that, the PHB says that tieflings aren't inherently evil, but tend to be chaotic in nature and often fall into neutral or evil alignment due to their rough upbringing.

With that, you can see that upbringing has more to do with alignment, but an inherently evil creature would be more prone to turn evil.

I think the confusion here is the difference between evil alignment, such as would be detected by a 3.5e "Detect Alignment" spell and absolute evil, such as would be detected by a 5e "Detect Evil" spell.

Check out 5th edition wording on Detect Evil:

For the duration, you know if there is an aberration, celestial, elemental, fey, fiend, or undead within 30 feet of you,

And by comparison, let's jump all the way back to 2e:

Character alignment is not revealed under most circumstances: Characters who are strongly aligned, do not stray from their faith, and who are at least 9th level might radiate good or evil if they are intent upon appropriate actions.

By 5e, they have gotten VERY specific about what Detect Evil is doing. This sort of thing has been hashed out over time and was a lot muddier in earlier editions. "Detect Evil" in 5e is not reading minds or making judgement. It's not going to detect a really evil orc who eats babies. He's an orc, not a demon or undead, so he's not an "evil" that Detect Evil can detect. He could still set off a Glyph of Warding against evil alinhamento, though, since Glyph can be set to look at alignment. Two orcs could walk into a room and one could set it off and the other not because it's an alignment issue, not an "evil creature" issue.

So the answer to your question is "no, because evil alignment (a choice) should not be confused with absolute evil (not a choice)". This is made clearer in newer editions but was pretty fuzzy in older editions, which I think is why there's lingering confusion.

For sake of comparison, undead babies would be evil. Their alignment is evil AND their status as a creature is "evil" for purposes of Detect Evil. They don't have free will in this matter. (And let's all take a moment to remember Pet Semetary and shudder. Yikes.)

Druegar tend towards evil alignment but are not an absolute evil. Their babies would be unaligned.

(Of course now I'm picturing a scenario where a team of wizards runs a day care center using a lot of glyphs, trying to determine precisely when alignment happens....)

If this were true, it would be impossible for a player to play a good-aligned character from an Evil race.

And yet, we know for a fact that Volo's Races includes a number of "Evil" races, such as Goblins and Hobgoblins, as playable Player Characters.

We can conclude, therefore, that there can exist members of such races that are not evil.

Your question specifically is asking about a baby of said race - a baby would have roughly the intelligence of an animal*, and Animals are not intelligent enough to have an alignment (unless trained - which a baby would not be). So, statistically, they do not have the intelligence to tem an alignment either.

Ultimately though, infants are not something covered by the CORE rules - so this is largely up to the DM's discretion. However, most of the rules lean in the direction of "No, a baby is not evil, regardless of race". And, additionally, it is possible for such a baby to grow up into someone who is tb not evil - such as a Player Character, or a very unique Non-Player character.

*In terms of ability score - not to say anything about babies in general.