Learn Hardcore Swahili: Watoto wengine wanapenda chai, na wengine wanapenda maziwa. - Some children like tea, and others like milk.

Mtoto means child (singular). Watoto is its irregular plural, meaning children.
In Swahili, many human nouns use the m-/wa- class:

mtoto = child
watoto = children

Because we’re talking about more than one child, the plural watoto is required.

What exactly does wengine mean in this sentence?

Wengine comes from the root -ingine, meaning other or another.
It can mean:

other (children) when used after a noun: watoto wengine = other children / some children
others when it stands alone: na wengine = and others / and the others

So in the sentence:

watoto wengine = some children / other children
na wengine = and others (the other children)

Why is it watoto wengine at the beginning, but just wengine (without watoto) after na?

In the second part, wengine stands for watoto wengine, and watoto is left out because it’s already clear from context.
Swahili often drops repeated nouns if the meaning is obvious.

If you want, you can say na watoto wengine wanapenda maziwa, but it’s more natural and less repetitive to say na wengine wanapenda maziwa.

Why is the verb wanapenda used here, and not anapenda?

The subject is watoto (children), which is plural. The verb must agree with the subject, so it uses the plural subject marker wa-:

anapenda = he/she likes (subject marker a- = he/she)
wanapenda = they like (subject marker wa- = they)

Since watoto are they, you must say wanapenda.

What does the -na- in wanapenda mean? Could it just be wapenda?

Wanapenda is made of:

wa- = they
-na- = present tense (often “is/are doing”, also used for general truths)
-penda = like / love

Wapenda is not standard modern Swahili for this meaning; you need the -na- (or another tense marker).
So wanapenda is the normal way to say they like or they love here.

Why isn’t there a word like “some” before watoto? In English we say “Some children…”.

In Swahili, wengine already carries the idea of some / other(s).
So watoto wengine can naturally mean some children or other children, depending on context.

If you really want to emphasize some (but not all), you can also say:
Baadhi ya watoto wanapenda chai… = Some of the children like tea…
But in everyday speech, watoto wengine is usually enough.

Why is the word order watoto wengine, not wengine watoto?

Normal Swahili word order is:

Noun + adjectives/quantifiers: watoto wengine, watoto wawili, watoto wadogo

So watoto wengine (children other / other children) is correct.
Putting wengine before the noun (wengine watoto) sounds wrong or very unnatural in standard Swahili.

Why is there no word for “the” or “some” before chai and maziwa?

Swahili has no articles like the, a, or some. Context tells you whether you should understand tea, the tea, some tea, etc.

So wanapenda chai can mean:

they like tea
they like the tea
they like some tea

The same applies to maziwa (milk).

Why is maziwa used for “milk” if it looks plural?

Maziwa literally belongs to a plural noun class (it’s the plural of ziwa in other meanings), but when it means milk, it behaves like a mass noun in English.

So:

maziwa = milk (as a substance)
You don’t normally use a different singular form for “milk”; maziwa covers it.

Even though it looks grammatically plural, you usually just translate it as milk.

Could I say Watoto wengine hupenda chai instead of wanapenda chai? What’s the difference?

Yes, you can say watoto wengine hupenda chai. The difference:

wanapenda = present tense; can mean they like, they are liking, or they usually like, depending on context
hupenda = habitual aspect; more specifically they usually/typically like

In everyday conversation, wanapenda is very common and perfectly natural here. Hupenda sounds a bit more formal or emphasizes a general habit.

Is the comma before na necessary: chai, na wengine?

The comma is stylistic, not a strict grammatical rule. You can write:

Watoto wengine wanapenda chai na wengine wanapenda maziwa.
Watoto wengine wanapenda chai, na wengine wanapenda maziwa.

Both are acceptable. The comma just gives a small pause and makes the contrast between the two groups clearer in writing.

How would I say “Some children don’t like tea, and others don’t like milk” using this structure?

You negate the verb penda with ha-…-i in the present tense:

hawapendi = they don’t like (ha- + wa- + -pend- + -i)

So you can say:

Watoto wengine hawapendi chai, na wengine hawapendi maziwa.
= Some children don’t like tea, and others don’t like milk.