Djeca su se sakrila iza kuće kad je počela kiša.

Because djeca takes neuter plural agreement in Croatian. The past participle form that matches neuter plural is -la.

So:

• djeca su se sakrila = the children hid
• mala djeca = small children

To an English speaker, sakrila may look singular, but here it is the correct plural agreement form for djeca.

Su is the 3rd person plural form of biti = to be, used here as an auxiliary verb to form the past tense.

So:

• su sakrila = hid / have hidden

Croatian past tense is commonly built with:

• a form of biti
• plus a past participle

Here:

• djeca = subject
• su = auxiliary
• sakrila = past participle

Se is a reflexive clitic. In this sentence it is part of the verb sakriti se = to hide oneself / to hide.

English usually just says hide, but Croatian often uses se with this verb:

• sakriti = hide something
• sakriti se = hide oneself / hide

So Djeca su se sakrila means The children hid.

The dictionary forms are:

• sakriti se = to hide
• početi = to begin

In the sentence, they appear in past forms:

• sakrila agrees with djeca
• počela agrees with kiša

Because iza requires the genitive case, and the genitive singular of kuća is kuće.

So:

• kuća = house, nominative
• kuće = of the house / house, genitive

That is why:

• iza kuće = behind the house

Because iza normally takes the genitive, not the accusative.

English learners often expect a movement/location contrast like:

• u grad vs u gradu
• na stol vs na stolu

But with iza, standard Croatian uses genitive:

• iza kuće = behind the house

The idea of movement comes from the verb sakrila se, not from changing the case after iza.

Kad means when. Yes, you can also say kada.

Both are correct:

• kad je počela kiša
• kada je počela kiša

Kad is shorter and very common in everyday Croatian.

Because je is a clitic, and Croatian clitics usually stand near the beginning of the clause.

So the normal order is:

• kad je počela kiša

Not:

• kad kiša je počela

The same thing happens in the main clause:

• Djeca su se sakrila

Here su and se also appear early, in the usual clitic position.

Because the sentence describes completed events.

• sakriti se is perfective: to hide, to get hidden
• početi is perfective: to begin, to start

The meaning is:

• the rain started
• the children hid

That fits a sequence of finished actions. If you used imperfective verbs, the meaning would change to something more ongoing, repeated, or process-like.

Croatian has no articles. There is no direct equivalent of English the and a/an in ordinary usage.

So iza kuće can mean:

• behind the house
• behind a house

Context tells you which meaning is intended. In this sentence, English naturally uses the house.

Yes. Croatian word order is fairly flexible.

For example, you can also say:

• Kad je počela kiša, djeca su se sakrila iza kuće.

That sounds natural too. But even when word order changes, the clitics still need to stay in their normal position:

• su se
• je

So you can move larger parts of the sentence around, but you cannot place the clitics just anywhere.