Learn Hardcore Malay: Anak kecil itu bermain bola di pinggir taman. - That small child plays ball at the edge of the park.

Just anak is neutral (child), while anak kecil adds the idea that the child is still small/young.
You can think of it like English “child” vs “little child / small child”.

Why is itu placed after anak kecil (as in anak kecil itu) instead of before, like English “that small child”?

In Malay, demonstratives like ini (this) and itu (that) usually come after the noun phrase, not before it.

anak kecil itu = that/the little child
anak kecil ini = this little child

So the pattern is:

noun (+ adjective) + itu/ini
not
itu/ini + noun

You wouldn’t normally say itu anak kecil in this context; that sounds marked or needs special emphasis and a different structure.

Does anak kecil itu mean “the little child” or “that little child”? What’s the nuance?

Itu literally means that, but in Malay it is also used very often where English uses the.

So anak kecil itu can be:

that little child (if you’re contrasting or pointing: not this one, that one over there)
the little child (referring to a specific child already known in the context)

In practice, the exact English translation depends on context and emphasis.
Malay doesn’t always distinguish as sharply between the and that as English does.

Why is there no word for “the” or “a” in this sentence?

Malay normally doesn’t use separate words for a/an or the.
Definiteness (whether something is specific or not) is understood from context, or by using words like:

itu / ini (that/this)
seorang, seekor, sebiji, etc. (classifiers, often translated as “a/an” in some contexts)

So:

anak kecil itu ≈ the / that little child
seorang anak kecil ≈ a little child (with seorang as a human classifier)

The original sentence doesn’t need separate words for the or a; Malay simply doesn’t mark articles the way English does.

Is anak kecil a fixed expression, or could it be kecil anak like in some other languages?

The normal order in Malay is noun + adjective, not the other way around.

anak kecil (correct) = small child
kecil anak (wrong/unnatural in this meaning)

So:

rumah besar = big house
baju merah = red shirt
bola kecil = small ball

Just remember: noun first, then adjective.

What exactly does bermain bola mean, and why do we use bermain instead of just main?

Bermain is the verb to play (intransitive), made from the root main with the prefix ber-.

bermain = to play (more standard/formal)
bola = ball
bermain bola = to play ball / to play with a ball (often implies playing a ball game, e.g. football)

In everyday spoken Malay, people often drop ber- and just say:

Anak kecil itu main bola.

Both are correct in meaning, but bermain bola sounds more standard or formal, while main bola is more colloquial.

Is there any tense marking here? How do I know if it’s “is playing”, “was playing”, or “will play”?

Malay verbs usually don’t change form for tense. Bermain stays the same for past, present, or future.
The sentence Anak kecil itu bermain bola di pinggir taman could be translated as:

The little child is playing ball at the edge of the park.
The little child was playing ball at the edge of the park.
The little child will play ball at the edge of the park.

Context (or extra time words) tells you the tense:

tadi (earlier), semalam (last night) → past
sedang (in the middle of doing) → present/progressive
nanti, esok (tomorrow) → future

Example:

Tadi, anak kecil itu bermain bola di pinggir taman. = The little child was playing ball at the edge of the park earlier.

What does di pinggir taman mean exactly, and why is the word order like that?

Di = at/in/on (location preposition)
pinggir = edge, side, margin
taman = park/garden (depending on context)

So:

di pinggir taman = at the edge of the park / by the side of the park

The structure is:

di + [location noun phrase]
pinggir taman literally = edge (of) park

You wouldn’t say di taman pinggir for this meaning; it must be di pinggir taman.

What’s the difference between di taman and di pinggir taman?

di taman = in the park / at the park (somewhere inside or within the park)
di pinggir taman = at the edge of the park / by the side of the park (near the boundary)

So di pinggir taman is more specific: it tells you the child is near the border of the park, not in the middle.

Could anak kecil itu mean “the little children” (plural), or is it always singular?

Grammatically, anak kecil itu is ambiguous in number; Malay doesn’t always mark plural.

It could mean:

the little child (one child)
the little children (more than one child)

Usually, context tells you which one is meant.
If you want to make plural very explicit, you can say:

anak-anak kecil itu = the little children
(reduplication anak-anak signals plural)

For clearly singular, you can use a classifier:

seorang anak kecil itu is odd; normally you’d say either
- seorang anak kecil = a/one little child
- anak kecil itu = the/that little child

Is di here the same di- that’s used as a passive prefix on verbs?

The di in di pinggir taman is a preposition meaning at / in / on. It is written separately from the word that follows.

The passive prefix di- is written attached to a verb:

dimakan = is/was eaten
ditulis = is/was written

So:

di pinggir taman (preposition, separate word)
vs.
dimakan, dibaca, ditendang (passive verbs, di- attached)

In your sentence, di is only a location preposition, not a passive marker.