Το παιδικό δωμάτιο είναι πιο ήσυχο όταν τα παιδιά κοιμούνται.

Here το παιδικό δωμάτιο means the children's room or the kids' room.

The adjective παιδικός, παιδική, παιδικό often means for children, related to children, or children's in a general descriptive sense. In this sentence, it describes the type of room.

It does not usually mean childish here. If you wanted to say that something is childish in the sense of immature, Greek would normally use a different word, such as παιδιάστικος.

Both are possible, but they are slightly different in feel.

• το παιδικό δωμάτιο = the children's room / the kids' room
  This is the normal, compact way to name the room as a type of room.

• το δωμάτιο των παιδιών = the room of the children / the children's room
  This sounds more like you are emphasizing whose room it is.

So παιδικό δωμάτιο is the more natural everyday phrase for children's room.

Because ήσυχο has to agree with δωμάτιο.

• δωμάτιο is a neuter singular noun.
• ήσυχος, ήσυχη, ήσυχο is an adjective, and Greek adjectives agree with the noun they describe in gender, number, and case.

So:

• το δωμάτιο → neuter singular
• ήσυχο → also neuter singular

That is why the sentence says είναι πιο ήσυχο and not πιο ήσυχος or πιο ήσυχη.

Yes. Greek commonly makes comparatives in two ways:

• πιο + adjective
• a one-word comparative form, often ending in -ότερος / -ότερη / -ότερο

So both of these are correct:

• πιο ήσυχο
• ησυχότερο

Both mean quieter.

In everyday Greek, πιο + adjective is very common and often feels simpler and more natural.

όταν means when.

It introduces a time clause:

• όταν τα παιδιά κοιμούνται = when the children sleep / when the children are sleeping

So the sentence structure is:

• main clause: Το παιδικό δωμάτιο είναι πιο ήσυχο
• time clause: όταν τα παιδιά κοιμούνται

The singular is το παιδί = the child.

The plural is τα παιδιά = the children.

This is a very common neuter noun:

• singular: το παιδί
• plural: τα παιδιά

Because it is neuter plural, it takes the article τα, not οι.

Yes, it has a middle/passive-style ending, but the verb itself means to sleep.

The basic verb is:

• κοιμάμαι = I sleep / I am sleeping

So:

• κοιμούνται = they sleep / they are sleeping

This is normal in Greek. Some verbs use middle/passive forms but have an active meaning.

Compare:

• κοιμάμαι = I sleep
• κοιμίζω = I put someone to sleep

So here κοιμούνται does not mean a true passive like they are being slept. It simply means they are sleeping.

It can mean either, depending on context.

Greek present tense often covers both:

• they sleep
• they are sleeping

In this sentence, English could translate it in two natural ways:

• The children's room is quieter when the children sleep.
• The children's room is quieter when the children are sleeping.

If the sentence is talking about a general pattern, sleep works well. If it describes what is happening at a particular time, are sleeping works well.

Yes. Greek word order is more flexible than English word order.

You could also say:

• Όταν τα παιδιά κοιμούνται, το παιδικό δωμάτιο είναι πιο ήσυχο.

This means the same thing: When the children are sleeping, the children's room is quieter.

The original order is perfectly natural. Changing the order mostly changes emphasis, not the basic meaning.

Greek uses the definite article more often than English does.

So τα παιδιά can often be natural even where English might simply say children, especially in general statements or when the group is understood from context.

That means Greek may sound more specific on the surface, but in many cases it is just normal Greek usage rather than extra emphasis.