Όταν το μωρό κοιμάται στο κρεβατάκι του, η μαμά του χαλαρώνει λίγο.

Όταν means when.

It introduces a time clause:

• Όταν το μωρό κοιμάται στο κρεβατάκι του = When the baby sleeps / is sleeping in its little bed

After that comes the main clause:

• η μαμά του χαλαρώνει λίγο = his mom relaxes a little

So the structure is:

Όταν + subordinate clause, main clause

This is very common in Greek.

Μωρό is a neuter noun in Greek, so it takes:

• the neuter article το
• neuter agreement where needed

So:

• το μωρό = the baby

This is grammatical gender, not necessarily biological sex. Even if the baby is a boy or a girl, the noun μωρό is still neuter.

Κοιμάται is the 3rd person singular present form of κοιμάμαι, which means to sleep.

So:

• κοιμάμαι = I sleep / I am sleeping
• κοιμάται = he/she/it sleeps / is sleeping

A useful point: κοιμάμαι looks like a middle/passive verb in form, but here it has an active meaning: sleep, not be slept.

It can mean either, depending on context.

Greek present tense often covers both:

• sleeps as a general/habitual action
• is sleeping as something happening now

In this sentence, both are possible in principle:

• When the baby sleeps in its little bed, his mom relaxes a little
• When the baby is sleeping in its little bed, his mom relaxes a little

The broader context would tell you which feels more natural.

Στο is a contraction of:

• σε = in, at, to
• το = the

So:

• σε το → στο

This is standard in Modern Greek.

Here:

• στο κρεβατάκι = in the little bed / crib

Similar contractions:

• σε + τη(ν) → στη(ν)
• σε + τα → στα

Κρεβατάκι is the diminutive form of κρεβάτι.

• κρεβάτι = bed
• κρεβατάκι = little bed

In context, it often suggests:

• a baby bed
• a crib
• a small bed

The ending -άκι is a very common Greek diminutive ending and can express small size, affection, or both.

Because Greek repeats the possessive pronoun with each noun it belongs to.

Here:

• στο κρεβατάκι του = in his/its little bed
• η μαμά του = his mom

So both του refer to the baby.

This is very natural in Greek. English might sometimes avoid repeating his, but Greek often keeps it.

Here του is a weak possessive pronoun meaning:

• his
• sometimes its

It comes after the noun in Greek:

• η μαμά του = literally the mom his
• natural English: his mom

A key point: του is the form used for a masculine or neuter singular possessor.

So because μωρό is neuter, Greek uses του.

Compare:

• ο μπαμπάς του = his dad
• το παιχνίδι του = his/its toy
• η μαμά της = her mom

Greek normally uses the definite article with specific nouns, including family words very often.

So:

• η μαμά του literally looks like the mom of him
• but it simply means his mom

This is completely normal Greek, not especially formal or unusual.

English usually says:

• his mom

Greek usually says:

• η μαμά του

The first part is a subordinate clause introduced by Όταν.

When that time clause comes first, Greek commonly separates it from the main clause with a comma:

• Όταν το μωρό κοιμάται στο κρεβατάκι του, η μαμά του χαλαρώνει λίγο.

This works much like English:

• When the baby is sleeping, his mom relaxes a little.

If the clauses were arranged differently, punctuation might also change.

Χαλαρώνει means relaxes or is relaxing.
Λίγο means a little.

So:

• χαλαρώνει λίγο = relaxes a little

The placement is natural and common. Greek adverbs are somewhat flexible, but this order sounds very normal.

You may also hear:

• λίγο χαλαρώνει

But in this sentence, χαλαρώνει λίγο is the most straightforward and natural choice.

Greek word order is fairly flexible, but not random. This sentence uses a very natural neutral order.

Current order:

• Όταν το μωρό κοιμάται στο κρεβατάκι του, η μαμά του χαλαρώνει λίγο.

You could rearrange parts for emphasis, for example:

• Η μαμά του χαλαρώνει λίγο όταν το μωρό κοιμάται στο κρεβατάκι του.

This still means the same thing, but the focus feels slightly different.

So the sentence is not rigidly fixed, but the original version is a very standard way to say it.