Djeca su se uplašila grmljavine.

Because djeca is a special noun.

Although it means children, it is grammatically treated as neuter plural in Croatian. The past participle therefore agrees with it in the neuter plural form:

• djeca su se uplašila = the children got frightened

The form uplašila looks the same as feminine singular, but here it is actually neuter plural. Croatian has several forms that happen to look identical.

A similar pattern appears with other verbs:

• Djeca su došla. = The children came.
• Djeca su zaspala. = The children fell asleep.

Se is a reflexive particle. In this sentence it is part of the verb uplašiti se, which means to get frightened / to become scared.

So:

• uplašiti = to frighten (someone)
• uplašiti se = to get frightened

Compare:

• Grmljavina je uplašila djecu. = The thunder frightened the children.
• Djeca su se uplašila grmljavine. = The children got frightened of the thunder.

Here se does not literally mean themselves in the English sense. It helps make a different verb meaning.

Because grmljavine is the genitive singular form of grmljavina.

The basic form is:

• grmljavina = thunder

After uplašiti se, Croatian normally uses the genitive for the thing someone becomes frightened of.

So:

• uplašiti se + genitive
• uplašiti se grmljavine = to get frightened of the thunder

This is similar to other Croatian verbs that also require genitive after se, for example:

• bojati se psa = to be afraid of the dog

Because su and se are clitics: short, unstressed words that usually go near the beginning of the sentence, often in second position.

In this sentence:

• Djeca = first stressed word
• su se = clitics placed right after it
• uplašila grmljavine = the rest

So the normal order is:

• Djeca su se uplašila grmljavine.

This placement is very typical in Croatian. English speakers often want to place words more freely, but with clitics Croatian has strong word-order habits.

More naturally, it means got frightened.

That is because uplašiti se is a perfective verb: it describes a completed event or change of state.

So this sentence suggests a moment when the children became scared:

• Djeca su se uplašila grmljavine. = The children got frightened by the thunder.

If you want the more general meaning were afraid of thunder, Croatian would more naturally use:

• Djeca su se bojala grmljavine. or
• Djeca su se plašila grmljavine.

So uplašila se is more about the reaction happening, not a long-term fear.

The dictionary form is uplašiti se.

In the sentence, uplašila is the past participle used to make the past tense together with su:

• su = auxiliary verb
• uplašila = past participle

So:

• uplašiti se = infinitive
• su se uplašila = got frightened

A useful way to recognize it is:

• infinitive: uplašiti se
• past tense with djeca: djeca su se uplašila

Yes, and it means almost the same thing from a different angle.

• Djeca su se uplašila grmljavine.
  = The children got frightened of the thunder.

• Grmljavina je uplašila djecu.
  = The thunder frightened the children.

The first version focuses on the children’s reaction.
The second version focuses on the thunder as the cause.

Both are natural.

Because Croatian has no articles.

There is no direct equivalent of English the or a/an in ordinary Croatian grammar. Whether something is definite or indefinite is usually understood from context.

So:

• djeca can mean children or the children
• grmljavina can mean thunder or the thunder

In this sentence, the intended meaning is clear from context, so no article is needed.

Yes, but the meaning stays basically the same while the emphasis changes.

For example:

• Djeca su se uplašila grmljavine. = neutral order
• Grmljavine su se djeca uplašila. = emphasizes the thunder
• Uplašila su se djeca grmljavine. = more marked, possible in context

Even when the order changes, the forms still show who did what, because Croatian uses case and agreement much more than English does.

Still, for a learner, Djeca su se uplašila grmljavine is the safest and most natural neutral version.