Copiilor nu le este somn încă, pentru că se joacă în camera de zi.

Copiilor is the dative plural form of copil.

In this sentence, Romanian uses a structure that is literally like:

To the children, sleep is not yet...

So the children are not treated as the normal grammatical subject here. They are the experiencer of the state, and Romanian marks that with the dative:

• copiii = the children as subject/object
• copiilor = to the children / for the children

Compare:

• Copiii se joacă. = The children are playing.
• Copiilor le este somn. = The children are sleepy.
  Literally: To the children, sleep is.

Because Romanian very often uses a clitic pronoun together with the full noun. This is called clitic doubling.

So:

• copiilor = the full noun phrase, to the children
• le = the short unstressed pronoun, to them

Together:

• Copiilor le este somn.

This is the normal, natural Romanian pattern. Leaving out le would sound incomplete or much less idiomatic in standard Romanian.

This happens a lot with indirect objects:

• I-am dat Mariei cartea. = I gave Maria the book.
• literally: To Maria, I gave her the book.

In Romanian, nu normally comes before the verb phrase, and short unstressed pronouns like le stay right next to the verb.

So the normal order is:

• nu le este

not something like:

• le nu este

This order is very common:

• nu îi place = he/she does not like
• nu le spun = I do not tell them
• nu mi-e foame = I am not hungry

So nu le este somn is exactly the expected pattern.

Because the verb is not agreeing with copiilor in the way an English learner might expect.

In this expression, Romanian uses the noun somn as part of an impersonal-style structure:

• îmi este somn
• îți este somn
• le este somn

The verb stays singular because the pattern is built around somn, not around copiilor as a normal plural subject.

So even with plural people, Romanian says:

• Copiilor le este somn.
• Părinților le este frig.
• Oamenilor le este foame.

This is a very common Romanian pattern for physical states and sensations.

Somn literally means sleep, but in the expression a-i fi somn, it means to be sleepy.

So:

• Mi-e somn. = I am sleepy.
• Îți este somn? = Are you sleepy?
• Le este somn. = They are sleepy.

There is no article because this is a fixed idiomatic expression. Romanian often uses bare nouns in these state expressions:

• mi-e foame = I am hungry
• mi-e sete = I am thirsty
• mi-e frig = I am cold
• mi-e somn = I am sleepy

So you should learn a-i fi somn as a set phrase.

Here încă means yet.

Because the sentence is negative:

• nu le este somn încă = they are not sleepy yet

A useful thing to remember is:

• in negative sentences, încă often means yet
• in positive sentences, încă often means still

For example:

• Nu am mâncat încă. = I haven’t eaten yet.
• Încă mănânc. = I am still eating.

You may also hear slightly different word order, such as:

• Copiilor încă nu le este somn...

That also sounds natural.

Because the Romanian verb for to play in the sense of children playing is usually a se juca.

So:

• copiii se joacă = the children are playing

This is a reflexive verb in form, even though in English we do not say play themselves.

It is important to distinguish:

• a se juca = to play
• a juca = usually to play/perform something, depending on context

For example:

• Copiii se joacă. = The children are playing.
• El joacă fotbal. = He plays football.
• Ea joacă un rol într-un film. = She plays a role in a film.

So in your sentence, se joacă is exactly the expected form.

Romanian often drops subject pronouns when they are not needed. This is very different from English.

So instead of saying:

• ei se joacă

Romanian often just says:

• se joacă

because the subject is understood from context.

In this sentence, the subject is clearly the children already mentioned earlier.

One small extra point: the form joacă can be both he/she plays and they play, so context matters here. Since the sentence is already about copiilor, the meaning is obviously plural.

You can say ei se joacă if you want emphasis or contrast, but it is not necessary.

În camera de zi means in the living room.

Literally, camera de zi is something like the day room:

• cameră = room
• camera = the room
• de zi = for daytime / day

So:

• camera de zi = the living room

It is camera and not cameră because it is definite:

• cameră = a room
• camera = the room

After în, both can appear depending on meaning:

• într-o cameră = in a room
• în camera de zi = in the living room

Yes. Pentru că and fiindcă can both mean because.

So you could say:

• Copiilor nu le este somn încă, pentru că se joacă în camera de zi.
• Copiilor nu le este somn încă, fiindcă se joacă în camera de zi.

Both are correct.

In everyday Romanian, pentru că is extremely common and very neutral. Fiindcă is also common, but many learners first encounter pentru că more often.

So for a beginner, pentru că is a very safe choice.