dàrén hé háizi dōu lái le.

Here 都 means both or all.

Because the sentence mentions two groups — 大人 and 孩子 — the most natural English sense is both:

大人和孩子都来了。
= Both the adults and the children came / have come.

With more than two things, 都 often translates as all.

In Chinese, 都 usually comes after the subject/topic and before the verb or adjective it affects.

So the pattern is often:

A 和 B + 都 + verb

That is exactly what happens here:

大人和孩子 + 都 + 来了

Even though 都 refers to 大人和孩子, its normal grammatical position is before the predicate, not directly attached to the noun phrase the way English both often is.

Here 了 shows that the action of coming has been completed or that a new situation has occurred.

So:

• 来 = come / to come
• 来了 = came / have come / has arrived

In this sentence, 了 helps give the sense that the adults and children have now arrived.

It often suggests a change of situation:

• before: they had not arrived
• now: they have arrived

来 by itself is just the verb to come.

来了 usually means the coming actually happened, so it can mean:

• came
• have come
• have arrived

Compare:

• 大人和孩子都来。
  This sounds incomplete or unnatural in most situations unless it is part of a larger context, such as a habitual statement.

• 大人和孩子都来了。
  This means they actually came / have arrived.

So in this sentence, 了 is very important.

Yes, the literal pieces are big + person, but as a word, 大人 means adult.

In modern Chinese, 大人 is commonly used when contrasting adults with children, especially in everyday contexts:

• 大人 = adults
• 孩子 = children

So 大人和孩子 is a very natural pairing, like adults and children.

A more formal word for adult is 成人, but 大人 fits this sentence very well.

Chinese nouns often do not need a special plural ending.

In this sentence, plurality is already clear because:

• there are two groups joined by 和
• 都 shows that both groups are included

So:

• 大人 here means adults
• 孩子 here means children

Adding 们 is usually unnecessary here.

Chinese often leaves number to context much more than English does.

Yes. 孩子 can be singular or plural depending on context.

Examples:

• 一个孩子 = one child
• 两个孩子 = two children
• 孩子来了 = the child came / the children came, depending on context

In 大人和孩子都来了, it is naturally understood as children because the sentence is talking about groups.

Also, the second syllable is usually neutral tone: háizi.

Here 和 means and, joining two nouns:

大人 和 孩子
= adults and children

That is a very standard use.

One thing to remember: 和 is mostly used to connect nouns or noun phrases. It is not always used exactly the way English and is used in every kind of sentence.

But in this sentence, it works just like and.

Chinese distinguishes between movement toward the speaker/reference point and movement away from it.

• 来 = come, toward here
• 去 = go, away from here

So 都来了 means that the adults and children came here / arrived here.

If you said 都去了, that would mean they went there instead.

Yes. 孩子和大人都来了 is grammatically fine.

It means the same basic thing: Both the children and the adults have come.

The order can change depending on:

• what sounds more natural in context
• what the speaker wants to mention first
• emphasis

That said, 大人和孩子 is a very common order, much like saying adults and children in English.

Chinese does not have articles like the or a/an.

So 大人 and 孩子 do not need anything equivalent to the adults or the children.

Whether the meaning is:

• adults and children
• the adults and the children
• some adults and some children

depends on context.

In this sentence, the meaning is usually understood from the situation, not from an article word.

• 孩子 is pronounced háizi. The first syllable has second tone, and the second syllable is usually a neutral tone.
• 了 here is pronounced le, also with a neutral tone.

So the end of the sentence sounds like:

dōu lái le

not a full strong lè in this case.

That neutral pronunciation is very common in natural speech.

Yes. This is a very useful pattern:

A 和 B 都 + verb + 了

It means both A and B did something / have done something.

Examples:

• 爸爸和妈妈都来了。
  Both Dad and Mom came.

• 老师和学生都到了。
  Both the teachers and the students arrived.

• 男的和女的都喜欢。
  Both men and women like it.

So 大人和孩子都来了 is a great model sentence to learn from.