Kad oboje djece zaspu, nas dvoje ćemo napokon popiti čaj.

Oboje is a collective numeral meaning both. Croatian often uses collective numerals such as dvoje, troje, četvero, oboje with:

• children
• mixed-gender groups
• certain collective nouns

Since djeca means children, oboje djece is a very natural pattern.

Compare:

• oba djeteta = both children
• obje žene = both women
• oboje djece = both of the children

So oboje is not just another form of oba/obje; it belongs to a different counting pattern.

Because oboje requires the noun to be in the genitive plural.

So:

• djeca = children
• oboje djece = both of the children

The same thing happens with other collective numerals:

• dvoje djece = two children
• troje djece = three children
• petero djece = five children

So djece is there because of the grammar of oboje.

Yes. Kad oba djeteta zaspu is also a perfectly natural sentence.

Both versions mean essentially the same thing:

• Kad oboje djece zaspu...
• Kad oba djeteta zaspu...

The difference is mostly grammatical style:

• oba djeteta uses the ordinary both + noun pattern
• oboje djece uses the collective numeral pattern, which is very common with children

So if oba djeteta feels easier to you, that is a good sentence too.

Zaspu is the 3rd person plural present of zaspati, which means to fall asleep.

Basic forms of zaspati:

• zaspim = I fall asleep
• zaspiš = you fall asleep
• zaspi = he/she falls asleep
• zaspimo = we fall asleep
• zaspite = you all fall asleep
• zaspu = they fall asleep

So oboje djece zaspu means both children fall asleep.

This is very normal in Croatian.

After time words like kad when you are talking about the future, Croatian usually uses the present tense, not a future form.

So:

• Kad oboje djece zaspu...
  means
• When both children fall asleep...

In English, you also often say when they fall asleep, not when they will fall asleep.

So the Croatian structure is very natural here.

Because these verbs do not mean the same thing:

• zaspati = to fall asleep
• spavati = to sleep / be sleeping

This sentence is about the moment when the children become asleep, not the ongoing state of sleeping.

So:

• Kad oboje djece zaspu = when both children fall asleep
• Kad oboje djece spavaju would suggest when both children are sleeping, which is a different idea

Croatian is being precise about the action.

Nas dvoje means the two of us.

Croatian often uses expressions like:

• nas dvoje = the two of us
• vas dvoje = the two of you
• njih dvoje = the two of them

This is a very common way to say that a group contains exactly two people.

So:

• nas dvoje ćemo... = the two of us will...

It emphasizes that it is specifically two people, not just we in a general sense.

With this kind of numeral expression, Croatian very often uses nas/vas/njih + dvoje.

So the standard idiomatic pattern is:

• nas dvoje
• vas dvoje
• njih dvoje

Even when the phrase is the subject of the sentence, nas dvoje is normal:

• Nas dvoje ćemo popiti čaj.

You may hear mi dvoje in speech, but nas dvoje is the form learners should recognize as the usual standard pattern.

Yes. Dvoje is especially natural when the two people are:

• mixed gender
• gender not specified
• treated simply as a pair

So nas dvoje works very well for the two of us when Croatian does not want to specify two males or two females.

You may also see more specific forms such as:

• nas dvojica = the two of us, both male
• nas dvije = the two of us, both female

But nas dvoje is the safe, general expression here.

Because ćemo is a clitic: a short unstressed word that usually goes in the second position in the clause.

Here, the first unit is nas dvoje, so ćemo comes right after it:

• Nas dvoje ćemo napokon popiti čaj.

This is a very common Croatian word-order pattern.

The future tense here is formed with:

• ćemo
  • popiti

So the structure is simply:

• the two of us
  • will
    • drink

Because popiti is the perfective verb, while piti is imperfective.

Very roughly:

• piti = to drink, to be drinking
• popiti = to drink up, to have/drink completely, to drink as one complete event

In this sentence, the idea is a single completed action:

• once the children are asleep, the two adults will finally have their tea

That is why popiti fits so well.

Napokon means finally, at last.

It adds the feeling that this action has been delayed and the speakers have been waiting for it:

• first the children must fall asleep
• only then can the two adults finally drink their tea

So napokon gives the sentence a very natural emotional tone.

Both mean when.

• kad = shorter, very common
• kada = fuller form, sometimes a bit more formal or emphatic

In this sentence, either would work:

• Kad oboje djece zaspu...
• Kada oboje djece zaspu...

There is no important meaning change here.

Because Kad oboje djece zaspu is a subordinate clause placed before the main clause.

Croatian normally separates that kind of clause with a comma:

• Kad oboje djece zaspu, nas dvoje ćemo napokon popiti čaj.

This is similar to English punctuation when a when-clause comes first.

Čaj is the direct object, so it is in the accusative.

But for many inanimate masculine singular nouns, the accusative looks exactly like the nominative.

So:

• čaj = tea
• popiti čaj = to drink tea

That is why you do not see a different ending here. The case is still accusative; it just happens to look the same as the basic form.