Die Kinder lesen die Geschichte abwechselnd vor.

Because the verb is vorlesen, which is a separable verb.

• Base verb: lesen = to read
• Separable prefix: vor-
• Full verb: vorlesen = to read aloud

In a normal main clause, the conjugated part goes in the second position, and the prefix moves to the end:

• Die Kinder lesen die Geschichte abwechselnd vor.

So lesen is the conjugated verb, and vor is separated and placed at the end.

Lesen means to read in general.

Vorlesen means to read aloud, usually so that other people can hear.

So:

• Die Kinder lesen die Geschichte. = The children read the story.
• Die Kinder lesen die Geschichte vor. = The children read the story aloud.

In your sentence, the meaning is specifically reading aloud, not just silent reading.

Because the subject is die Kinder, which is plural.

Present tense of lesen:

• ich lese
• du liest
• er/sie/es liest
• wir lesen
• ihr lest
• sie/Sie lesen

Since die Kinder = the children, you use the plural form lesen.

They belong to two different nouns:

• die Kinder = the children
• die Geschichte = the story

Even though both articles are written as die, they are doing different jobs:

• die Kinder is nominative plural and is the subject
• die Geschichte is accusative singular feminine and is the direct object

German articles often look the same in different cases, so you have to identify their function from the noun and the sentence structure.

It is in the accusative case because it is the direct object of the verb.

Ask yourself: what are the children reading aloud?

Answer: die Geschichte

That makes die Geschichte the thing receiving the action, so it is accusative.

Because Geschichte is feminine, the article is die in both nominative and accusative singular:

• nominative: die Geschichte
• accusative: die Geschichte

So the form does not change here, even though the case is accusative.

Abwechselnd means taking turns, alternately, or one after another.

In this sentence, it tells you how the children are reading aloud: not all at once, but by taking turns.

So the idea is something like:

• one child reads a bit
• then another child reads
• then another

It functions as an adverb here.

In this sentence, abwechselnd is in a very natural position:

• Die Kinder lesen die Geschichte abwechselnd vor.

It usually goes in the middle field of the sentence, before the separable prefix at the end.

It can sometimes be moved for emphasis, but not every position sounds equally natural. For example:

• Die Kinder lesen abwechselnd die Geschichte vor.
  This is possible, but it puts slightly more focus on taking turns.

The original version is very normal and idiomatic.

Yes. Die Geschichte means the story, so it refers to a specific story that is already known from the context.

• die Geschichte = the story
• eine Geschichte = a story

If you said Die Kinder lesen eine Geschichte abwechselnd vor, that would mean they are reading some story / a story aloud by taking turns, not a specific one already identified.

The structure is:

• Die Kinder = subject
• lesen = conjugated verb
• die Geschichte = direct object
• abwechselnd = adverb
• vor = separable prefix

So the pattern is roughly:

Subject + finite verb + object + adverb + separable prefix

This is a very common pattern in German main clauses with separable verbs.

Good question. In German, separable verbs behave differently depending on the structure.

In a main clause

The prefix separates:

• Die Kinder lesen die Geschichte vor.

In an infinitive

The verb stays together:

• Die Kinder wollen die Geschichte vorlesen.

In a subordinate clause

The verb usually goes to the end and stays together:

• ..., weil die Kinder die Geschichte abwechselnd vorlesen.

So yes, vorlesen is one verb, but in main clauses it gets split.

No, not with abwechselnd.

Abwechselnd clearly means they are taking turns, so the sentence does not suggest that they are all reading simultaneously.

Without abwechselnd, the sentence could be less specific:

• Die Kinder lesen die Geschichte vor.

That could simply mean the children are reading the story aloud, without telling you exactly how they are doing it. Adding abwechselnd removes that ambiguity.