Ako se u nedjelju otopli, ići ćemo kamo god djeca požele, možda čak i na obalu.

Here se is part of the verb otopliti se, which means to get warmer / to warm up.

In weather expressions, Croatian often uses this kind of verb without an explicit subject:

• Otoplilo se. = It got warmer.
• Ako se otopli... = If it gets warmer...

English needs dummy it, but Croatian does not. So se is not literally itself here; it is just part of how the verb is formed.

Because after ako (if), Croatian normally uses the present tense, even when the meaning is future.

So:

• Ako se otopli... = If it gets warmer...
• not normally Ako će se otopliti...

Also, otopliti se is a perfective verb. Its present-tense form often refers to a future completed event:

• otopli = it gets warmer / it will have warmed up enough

This is actually similar to English:

• If it gets warmer, we'll go... not
• If it will get warmer, we'll go...

Because se is a clitic, and Croatian clitics usually go near the beginning of the clause, often in second position.

So the neutral order is:

• Ako se u nedjelju otopli...

This sounds natural. A version like Ako u nedjelju se otopli is not the normal neutral order.

It means on Sunday.

Here u is used with a time expression:

• u nedjelju = on Sunday
• u ponedjeljak = on Monday

This usually refers to a specific Sunday understood from context, often the coming one.

Useful contrast:

• u nedjelju = on Sunday / one specific Sunday
• nedjeljom = on Sundays / every Sunday

Because ćemo is also a clitic-like form and normally cannot stand at the very beginning of a clause by itself.

So when the clause begins with the verb phrase, Croatian naturally uses:

• ići ćemo = we will go

If some other word comes first, then ćemo can follow it:

• Možda ćemo ići... = Maybe we'll go...

So in your sentence, ići ćemo is the natural choice.

Kamo god means wherever in the sense of to whatever place or no matter where to.

It is made of:

• kamo = to where
• god = ever / no matter

So:

• kamo god djeca požele = wherever the children want

The little word god gives the idea of no matter where.

Because ići means to go, so the sentence needs a word of direction/destination, not location.

Compare:

• kamo = to where (destination)
• gdje = where (location)
• kuda = which way / along what route

So:

• ići kamo god... = go wherever... is correct
• gdje god would be more about being somewhere, not going there

Požele comes from poželjeti, which is a perfective verb. Here it suggests whatever the children happen to want / decide they want at that moment.

So:

• djeca žele = the children want (more general, ongoing)
• djeca požele = the children come to want / feel like / decide they want (more one-off, future-oriented)

In this sentence, požele fits the idea of a future decision:

• we'll go wherever the children feel like going

Yes. Croatian is leaving out ići because it is easy to understand from the context.

So:

• kamo god djeca požele literally means wherever the children want
• but the full idea is wherever the children want to go

A fuller version would be:

• ići ćemo kamo god djeca požele ići

That is grammatical, but the shorter version is very natural.

It means maybe even.

The parts are:

• možda = maybe
• čak i = even

So:

• možda čak i na obalu = maybe even to the coast

čak adds emphasis, and i helps form the very common expression čak i.

Obalu is accusative singular of obala (coast, shore).

It is accusative because na takes the accusative when there is movement toward a place:

• ići na obalu = to go to the coast

Compare:

• na obalu = to the coast (motion)
• na obali = on the coast / at the coast (location)

So the sentence uses na obalu because the speaker is talking about going there.

Yes, Croatian word order is fairly flexible, but not completely free.

This sentence is a natural neutral version:

• Ako se u nedjelju otopli, ići ćemo kamo god djeca požele, možda čak i na obalu.

Some parts can move for emphasis, for example:

• Ako se u nedjelju otopli, možda ćemo ići kamo god djeca požele...

But you should keep a few things in mind:

• clitics like se and ćemo have placement rules
• kamo god should stay together
• the comma after the ako-clause is normal

So yes, the order can change somewhat, but not randomly.