Børnene er ikke vant til at være stille så længe, så læreren giver dem en kort pause.

Because barn is an irregular noun.

• singular indefinite: et barn = a child
• plural indefinite: børn = children
• plural definite: børnene = the children

So børnene is the definite plural form of barn.

Læreren means the teacher. The definite form is used because the speaker is talking about a specific teacher, not just any teacher.

• en lærer = a teacher
• læreren = the teacher

This matches English quite closely here.

In a normal Danish main clause, the finite verb usually comes first in the verb slot, and ikke usually comes after it.

So:

• Børnene er ikke vant ...

is the normal word order.

This is similar to many other Danish sentences:

• Han kommer ikke.
• Vi har ikke tid.

Vant means used to or accustomed to.

The expression at være vant til ... is very common in Danish and means to be used to ...

So:

• er ikke vant til ... = are not used to ...

Even though vant looks like a past participle historically, in modern Danish it functions like an adjective in this expression.

Because være vant til is a fixed expression, and if what follows is a verb, Danish normally uses:

vant til + at + infinitive

So:

• vant til at være stille = used to being / used to being quiet

This is different from English. English often says used to being quiet, with -ing, but Danish uses an infinitive:

• vant til at være stille

Here it means quiet.

After være, stille often means calm, silent, or quiet:

• være stille = be quiet

It does not mean motionless here. The classroom context makes quiet the natural meaning.

Here så længe means for that long or so long in the sense of duration.

So:

• at være stille så længe = to be quiet for that long

Be careful: så længe can also mean as long as in other contexts, but not here. In this sentence it is about duration.

No, the two så words do different jobs.

1. så længe
   Here så means so/that in the sense of degree or extent:

   • så længe = that long / so long

2. ..., så læreren giver ...
   Here så means so, introducing a result:

   • The children are not used to it, so the teacher gives them a short break.

So the same word appears twice, but with different functions.

Because this så is a coordinating conjunction meaning so, connecting two main clauses.

After this kind of så, Danish keeps normal main-clause word order:

• så læreren giver dem en kort pause

If så were being used differently, for example as a fronted adverb meaning something like then, inversion could happen in other sentence types. But here it is simply linking two clauses, so subject + verb stays normal:

• læreren giver

Because dem is the object form of they.

• de = they
• dem = them

In this sentence, the teacher is giving something to them, so Danish uses the object form:

• læreren giver dem en kort pause

Because Danish usually puts the indirect object before the direct object when both come after the verb.

So the natural order is:

• giver dem en kort pause
  literally: gives them a short break

This is the most idiomatic pattern.

You can sometimes use til dem, but it is less neutral here and often sounds more marked:

• giver en kort pause til dem

The version in the sentence is the normal everyday choice.

Because pause is a common gender noun in Danish, so it takes en.

• en pause = a break
• pausen = the break

Since kort modifies an en-word, it appears as:

• en kort pause

If it were an et-word, the adjective form would often be different, for example et kort ord.

Because the sentence contains two main clauses:

1. Børnene er ikke vant til at være stille så længe
2. så læreren giver dem en kort pause

In written Danish, a comma is normally used to separate main clauses like this. So the comma helps show the structure clearly.

Yes. It works together as the complement of vant til.

You can think of it like this:

• Børnene = subject
• er = finite verb
• ikke vant til = are not used to
• at være stille så længe = to being quiet for so long / to be quiet that long

Inside that part:

• at være = to be
• stille = quiet
• så længe = for so long

So it is a single infinitive phrase after vant til.