Learn Hardcore Portuguese (Portugal): A ambulância passou e a sirene assustou as crianças. - The ambulance went by and the siren scared the children.

They are in the Preterite (Pretérito Perfeito Simples), 3rd person singular: ele/ela passou, ele/ela assustou. This tense expresses completed events in the past, so the sentence reports two finished actions: the ambulance went by, and the siren frightened the children. Using the Imperfect (passava, assustava) would suggest habitual or ongoing past actions.

Who is the subject in the second clause? The ambulance or the siren?

The subject is a sirene. Portuguese repeats the noun and article to mark the new subject: A ambulância passou e a sirene assustou... If you wanted the ambulance to be the subject of both verbs, you could say A ambulância passou e assustou as crianças.

Why do we use definite articles (a, as) everywhere? Can I omit them?

In European Portuguese, definite articles are used with specific, identifiable nouns by default, even more often than in English. So A ambulância, a sirene, as crianças is normal. Dropping the articles (e.g., Ambulância passou...) sounds like a headline or very telegraphic style.

Are ambulância, sirene, and criança feminine nouns? Why is it as crianças even for mixed groups?

Yes. ambulância and sirene are feminine nouns, so they take a (singular) and as (plural). criança is grammatically feminine, so the plural is as crianças even if the group includes boys. The singular is a criança.

How do I say “its siren” clearly in European Portuguese?

You can make possession explicit in a few ways:

A sirene da ambulância (the ambulance’s siren; note the contraction de + a = da).
a sirene dela (its siren; literally “the siren of her/it”). This avoids the ambiguity of sua in EP.
a sua sirene is also possible, but in EP seu/sua can ambiguously refer to someone else already in the context, so many speakers prefer dele/dela for clarity.

Can I replace as crianças with a pronoun? Where does it go?

Yes. In EP, in affirmative main clauses the object pronoun goes after the verb (enclisis) with a hyphen:

A sirene assustou-as. With negatives or other “attractors,” it goes before the verb (proclisis):
A sirene não as assustou.
A sirene já as assustou.

Should there be a comma before e?

No comma is needed here. It’s a simple coordination of two clauses with e. In Portuguese, you only use a comma before e in special cases (parenthetical insertion, contrast, repeated subject clarifications, etc.), which don’t apply here.

Does passar need a preposition, like por? Why is it just passou?

It can be either:

Intransitive: A ambulância passou = “The ambulance went by.”
With por: A ambulância passou por aqui/por nós = “passed by here/by us.” Your sentence uses the intransitive form; the idea of “by” is understood from context.

What’s the difference between assustar and assustar-se here?

assustar alguém = “to frighten someone” (transitive): A sirene assustou as crianças.
assustar-se (com) = “to get frightened (by/with)” (reflexive): As crianças assustaram-se com a sirene. Both are correct; they just focus on different participants as the subject.

Any quick pronunciation tips for the tricky words?

ambulância: stress on the lân syllable; roughly “ahm-bu-LAN-syah.”
sirene: “see-REH-neh.”
crianças: “kree-AHN-sas.” The ã is a nasal “an” sound, and ç is always an “s” sound.

Can I invert the order and say Passou a ambulância e a sirene assustou as crianças?

Yes, EP allows subject–verb inversion for stylistic or discourse reasons (especially in narratives). Passou a ambulância... is acceptable but more marked; the neutral order is A ambulância passou... The meaning stays the same; inversion emphasizes the verb/action or fits a storytelling rhythm.

Why is it assustou as crianças and not assustou às crianças?

Because assustar takes a direct object. No preposition is used: assustar alguém. You’d use às (a + as) with verbs that take an indirect object, e.g., dar algo às crianças (“give something to the children”).

How would I say this if it happened repeatedly or habitually?

Use the Imperfect:

A ambulância passava e a sirene assustava as crianças. You can also add a temporal frame: Sempre que a ambulância passava, a sirene assustava as crianças.

Is the sentence equally natural in Brazil? Any differences I should know?

It’s perfectly understandable in Brazil. Two notes:

Object pronouns: in Brazil, enclisis like assustou-as is rare in speech; they keep the noun (assustou as crianças) or use proclisis in contexts that allow it (e.g., não as assustou).
Pronunciation differs, but the spelling and grammar of your sentence are fine in both varieties.

Could I use alarme instead of sirene? What’s the difference?

sirene refers to the wailing device/sound on vehicles.
alarme is broader: a device/system or the event of an alarm going off.
For an emergency vehicle’s wail, sirene is the precise word: A sirene assustou as crianças.

Why is it a sirene and not à sirene?

a here is the feminine singular definite article (“the”). à is the contraction of the preposition a + article a (“to the”). Since a sirene is the subject (not an object of a preposition), no contraction is needed: a sirene, not à sirene.