Learn Hardcore Portuguese (Brazil): Quando minha mãe está em reunião, é bom que as crianças fiquem quietas. - When my mother is in a meeting, it is good that the children stay quiet.

Because fiquem is in the present subjunctive, and Portuguese uses the subjunctive after impersonal expressions like é bom que, é importante que, é necessário que, etc.

ficam = present indicative (simple statement of fact: the children stay/are quiet).
fiquem = present subjunctive (wish, recommendation, evaluation: it’s good that the children stay quiet / the children should stay quiet).

So é bom que as crianças fiquem quietas expresses an opinion/recommendation, not just a neutral fact, which is why the subjunctive is required.

What does é bom que mean, and why do we need que there?

É bom que literally means “it is good that”.

é bom = it is good
que introduces the clause that you are evaluating: que as crianças fiquem quietas (that the children stay quiet).

In Portuguese, many expressions of opinion/necessity follow this pattern:

É importante que você chegue cedo.
É melhor que ele vá agora.

So que is needed to introduce the subordinate clause and trigger the subjunctive (fiquem).

Could we say É bom as crianças ficarem quietas instead? Is that wrong?

It’s not wrong; it’s possible and used in Brazilian Portuguese:

É bom que as crianças fiquem quietas.
More “standard” and slightly more formal; uses subjunctive.
É bom as crianças ficarem quietas.
Uses the infinitive (ficarem) with an explicit subject (as crianças).

The meaning is practically the same. For learners, it’s very safe and useful to master the pattern é bom que + subjunctive, because it works with many verbs and is always clearly correct.

Why is quietas feminine? Aren’t “children” mixed boys and girls?

In Portuguese, adjectives agree with the grammatical gender of the noun, not with the actual gender mix of the people.

criança is a feminine noun in Portuguese (even if the child is a boy).
Plural: crianças (feminine plural).
Therefore the adjective must also be feminine plural: quietas.

So:

as crianças quietas (correct)
as crianças quietos (incorrect, because quietos is masculine plural).

Why do we say as crianças and not just crianças?

The definite article as makes it clear we’re talking about a specific group of children (in context, probably “her children” or “the children who are there”).

as crianças = the children (specific group)
crianças (without article) sounds more like children in general or part of a more generic phrase.

In this sentence, the situation is clearly specific (when my mother is in a meeting, the particular children around should be quiet), so as crianças sounds natural.

Could we say é bom que crianças fiquem quietas without the article?

That would sound strange and incomplete in this context. Without as, it feels like you mean “it’s good that children (some children, in general) stay quiet” but with no clear reference.

Native speakers almost always say é bom que as crianças fiquem quietas here, with the article, to refer to the specific children involved in that situation.

Why is it minha mãe and not a minha mãe?

In Brazilian Portuguese, with close family members, it’s very common (and often more natural) to use possessives without the article:

minha mãe, meu pai, minha irmã, meu filho, etc.

In European Portuguese, people often say a minha mãe, o meu pai, etc., but in Brazil the version without the article is extremely frequent and perfectly standard.

So Quando minha mãe está em reunião… is the usual Brazilian way to say it.

What does em reunião literally mean, and why em instead of na?

Literally, em reunião is “in (a) meeting”, but it’s used more like “in a meeting / in a meeting right now” as a state or activity.

Portuguese often uses em + noun (without article) to describe someone’s current activity or situation:

Ela está em reunião. – She is in a meeting.
Ele está em aula. – He is in class.
Eles estão em entrevista. – They are in an interview.

If you say na reunião (em + a reunião), it usually points to a specific meeting as an event or location:

Ela está na reunião das 3 horas. – She is at the 3 p.m. meeting.

In your sentence, you care about the activity/state (whenever she is in a meeting), so em reunião is the natural choice.

Could we say Quando minha mãe estiver em reunião instead of quando minha mãe está em reunião?

Yes, you could say Quando minha mãe estiver em reunião, é bom que as crianças fiquem quietas.

está (present indicative) is used here for a general or habitual situation: whenever that happens, the kids should be quiet.
estiver is future subjunctive, often used after quando, se, etc., when referring to a specific future time: when she is in a meeting (in the future).

Both are possible, but:

Quando minha mãe está em reunião… = more general/habitual.
Quando minha mãe estiver em reunião… = often sounds more like a future condition relative to something specific you’re talking about.

Why is there a comma after reunião?

The part Quando minha mãe está em reunião is an adverbial clause (it sets the condition/time). In Portuguese, when such a clause comes before the main clause, it is normally separated by a comma:

Quando minha mãe está em reunião, é bom que as crianças fiquem quietas.

If you invert the order, the comma is usually not used:

É bom que as crianças fiquem quietas quando minha mãe está em reunião.

Can we change the order of the clauses without changing the meaning?

Yes. You can say:

Quando minha mãe está em reunião, é bom que as crianças fiquem quietas.
É bom que as crianças fiquem quietas quando minha mãe está em reunião.

The meaning is the same. The first version emphasizes the condition/time (“Whenever my mother is in a meeting…”), while the second starts with the recommendation (“It’s good that the children stay quiet…”), but both are natural and correct.