Djeca nisu htjela ući u more jer je jedna meduza plutala uz obalu.

Djeca is the irregular plural of dijete (child).

So:

• dijete = child
• djeca = children

Even though djeca does not look like a typical plural noun, it is treated as plural in meaning and agreement.

With djeca, Croatian uses the neuter plural form of the past participle.

The verb here is built like this:

• nisu = they were not / did not (auxiliary part)
• htjela = past participle of htjeti in the neuter plural form

That is why you get:

• Djeca nisu htjela...

Useful comparison:

• htjeli = masculine plural
• htjele = feminine plural
• htjela = neuter plural

This neuter plural form happens to look the same as the feminine singular form, which can be confusing at first.

In Croatian, the negative forms of biti (to be) are fused into single words:

• nisam
• nisi
• nije
• nismo
• niste
• nisu

So in the perfect tense, you do not say ne su htjela.
You say:

• nisu htjela

This is just the standard grammatical form.

After htjeti (to want), Croatian normally uses an infinitive to express the action someone wants to do.

So:

• htjeti ući = to want to go in / to want to enter

That is why the structure is:

• nisu htjela ući = did not want to enter / go in

This is very similar to English wanted to enter.

This is about aspect.

• ući is perfective: to enter, to go in as a completed action
• ulaziti is imperfective: to be going in, to go in habitually/repeatedly

Here, the meaning is that the children did not want to go into the sea as a whole action. So ući is the natural choice.

If you used ulaziti, it would sound more like an ongoing or repeated process, which is not the point here.

Because they are doing two different jobs:

• ući is the verb itself: to enter / go in
• u is the preposition: into / in

So:

• ući u more = to enter into the sea

The first u is part of the verb.
The second u is a separate word showing direction.

You see the same pattern in sentences like:

• ući u kuću = enter the house
• ući u sobu = enter the room

Because Croatian distinguishes between motion toward/into a place and location in a place.

With u:

• u + accusative = motion into
• u + locative = location in

So:

• u more = into the sea → accusative
• u moru = in the sea → locative

Here the children do not want to go into the sea, so Croatian uses u more.

Compare:

• Djeca su u moru. = The children are in the sea.
• Djeca ulaze u more. = The children are going into the sea.

Because je is a clitic. Croatian clitics usually go very near the beginning of the clause.

After conjunctions like jer (because), the auxiliary clitic often appears immediately after the conjunction:

• jer je jedna meduza plutala...

That is the normal, natural word order here.

The version jer jedna meduza je plutala is much less natural in standard neutral Croatian.

Jedna literally means one, but it can also work a bit like a / one particular in English.

Here it suggests:

• one jellyfish
• a single jellyfish

That can add a slight nuance of just one jellyfish was enough to scare them off.

Without jedna, the sentence would still be grammatical:

• jer je meduza plutala uz obalu

But that version loses the small emphasis on one.

Because plutati and plivati are not the same thing.

• plutati = to float, drift
• plivati = to swim

A jellyfish in this sentence is presented as floating/drifting near the shore, not actively swimming.

Also, plutala is imperfective, which fits the idea of a background situation:

• the children did not want to go in
• because a jellyfish was floating near the shore

So plutala is a very good choice here.

The preposition uz means something like:

• along
• by
• up against
• sometimes near, depending on context

It takes the accusative case, so:

• obala → obalu

That is why the sentence has:

• uz obalu

In this context, it means the jellyfish was floating by / along the shore.

These mean different things.

• Djeca nisu htjela ući u more.
  = The children didn’t want to go into the sea.

• Djeca nisu ušla u more.
  = The children didn’t go into the sea.

The first sentence tells you about their unwillingness or reluctance.
The second just states the fact that they did not enter.

So htjela adds an important meaning that is not present in nisu ušla.

A rough guide for an English speaker:

• djeca sounds approximately like dye-tsa, but with a softer beginning than normal English d
• ući sounds roughly like oo-chee, but the ć is softer than English ch

A few key points:

• dj in djeca is not the same as English j in jam
• c in Croatian is always like ts
• ć is a soft sound, somewhat between t and ch for many English learners

So:

• djeca ≈ dye-tsa
• ući ≈ oo-chi, with a softer ch

These are only approximations, but they are useful starting points.