Kiedy dzieci mokną na boisku, mama mówi im, żeby wróciły do domu i suszyły skarpetki.

Both are possible, but they mean slightly different things.

• mokną = are getting wet / are becoming wet
• są mokre = are wet

So dzieci mokną na boisku focuses on the process: the children are out on the playground and getting wetter.
If you said dzieci są mokre na boisku, it would simply describe their state.

This is a very common distinction in Polish:

• być + adjective = state
• a verb like moknąć = change/process

Boisku is in the locative case.

After na, Polish can use:

• accusative for movement onto a place
• locative for being in/on a place

Here we have na boisku = on the playground / at the playing field, so it describes location, not motion.

Compare:

• na boisku = on the playground
• na boisko = onto the playground

So:

• dzieci mokną na boisku = the children are getting wet on the playground
• dzieci biegną na boisko = the children are running onto the playground

Because the verb mówić takes the dative case for the person you speak to.

• im = dative plural of oni / one
• ich is genitive/accusative, so it does not fit here

So:

• mówi im = she tells them / speaks to them

This is a useful pattern:

• mówić komuś = to speak to someone / tell someone
• dać komuś = to give someone
• pomagać komuś = to help someone

Examples:

• Mama mówi dzieciom...
• Mama mówi im...

Both mean Mom tells the children...

Żeby introduces a clause meaning something like:

• that
• so that
• in order that
• very often after verbs of telling, asking, wanting: to

In this sentence, mama mówi im, żeby... means:

• Mom tells them to...

So żeby is a very common way to express commands, wishes, requests, or intentions in a subordinate clause.

Examples:

• Chcę, żeby przyszli. = I want them to come.
• Proszę, żebyś usiadł. = I’m asking you to sit down.
• Mama mówi im, żeby wróciły... = Mom tells them to come back...

This is one of the things that often surprises English speakers.

After żeby, Polish normally uses forms that look like past tense plural/gender forms, but here they do not mean past time. They express something like a desired, requested, or intended action.

So:

• żeby wróciły = that they come back / to come back
• żeby suszyły = that they dry / to dry

The -ły ending appears because the subject is dzieci, which is grammatically treated as non-masculine plural.

You can think of it like this:

• wróciły in a normal sentence can mean they came back
• after żeby, wróciły means that they come back / should come back

So the form looks past, but the meaning is not past here.

Because dzieci is a plural non-masculine personal subject.

In Polish past-type verb forms agree with:

• number
• gender / gender class

For plural, there is an important split:

• masculine personal: groups containing males as persons → often -li
• non-masculine personal: things, animals, groups of females, mixed non-personal groups, and many nouns like dzieci → often -ły

So:

• chłopcy wrócili = the boys came back
• dziewczynki wróciły = the girls came back
• dzieci wróciły = the children came back

That is why we get:

• żeby wróciły
• żeby suszyły

This is about aspect.

• wrócić = perfective = to come back, to return (seen as a completed action)
• wracać = imperfective = to be returning / return habitually / return repeatedly

In this sentence, the mother is telling them to perform a specific completed action:

• go back home

So wróciły is natural.

If you used wracały, it would sound less natural here, because it would suggest an ongoing or repeated process rather than the single result get back home.

A good rough contrast:

• żeby wróciły do domu = so that they go back home
• żeby wracały do domu wcześniej = so that they come back home earlier (habitually / on repeated occasions)

Again, this is aspect.

• suszyć = imperfective = to dry, to be drying
• wysuszyć = perfective = to dry completely

Here suszyły skarpetki can sound more neutral, focusing on the activity: dry their socks.

If you said żeby wysuszyły skarpetki, it would emphasize the completed result more strongly:

• so that they dry the socks completely

In many contexts, both could work, but the nuance changes:

• suszyć = the action/process
• wysuszyć = the finished outcome

Polish often mixes aspects in one sentence depending on what is being emphasized:

• wróciły: first complete the action of returning home
• suszyły: then do the activity of drying the socks

Because do means to, into with motion toward a place, and it requires the genitive case.

• base form: dom
• genitive singular: domu

So:

• do domu = to home / homeward
• natural English translation: home

Very common examples:

• iść do szkoły = to go to school
• wrócić do pracy = to return to work
• jechać do Polski = to go to Poland

Because skarpetki is the accusative plural, used as the direct object of suszyć.

The basic singular is:

• skarpetka = sock

Plural:

• skarpetki = socks

In this case, for a non-masculine plural noun like skarpetki, the accusative plural looks the same as the nominative plural:

• skarpetki = socks
• suszyć skarpetki = to dry socks

So there is no special extra ending to learn here in this sentence.

In this sentence, tells is the better translation.

• mówi literally means speaks / says
• but with a person in the dative and a żeby clause, it often functions like tells

So:

• mama mówi im, żeby wróciły do domu = Mom tells them to go back home

If you translated it as Mom says to them that they should go home, it would be understandable, but less natural in English.

Yes, Polish word order is quite flexible.

This sentence uses a very natural neutral order:

• mama mówi im, żeby...

But you could also hear:

• Mama im mówi, żeby...
• Im mama mówi, żeby... — this puts stronger emphasis on them

Polish uses word order more flexibly than English because cases already show grammatical roles. Still, some orders sound more neutral than others.

The version in the sentence is a normal, unmarked one.

It can suggest either, depending on context.

• kiedy basically means when
• in sentences about regular situations, it can feel like whenever

So:

• Kiedy dzieci mokną na boisku, mama mówi im...

could mean:

• When the children get wet on the playground, Mom tells them... or
• Whenever the children get wet on the playground, Mom tells them...

If this is describing a repeated family situation, whenever may fit better in English.
If it is describing a single scene, when is enough.

Polish usually does not use the same structure as English here.

English:

• Mom tells them to come back home

Polish normally prefers:

• Mama mówi im, żeby wróciły do domu

That is:

• main verb
• person affected
• żeby clause

Polish can use infinitives in many places, but after verbs like tell, want, ask, order, a żeby clause is extremely common and often the most natural choice.

So this is a structure worth learning as a pattern:

• powiedzieć komuś, żeby...
• prosić kogoś, żeby...
• chcieć, żeby...

It is plural.

• singular: dziecko = child
• plural: dzieci = children

This is an irregular plural, so it does not follow the most basic noun pattern.

Because the subject is plural, the later verbs are plural too:

• dzieci mokną
• żeby wróciły
• żeby suszyły

So even though dzieci may not look like a familiar plural ending to an English speaker, it definitely means children.