Troje djece iz naše zgrade danas se igralo u parku; oboje mlađih ide u vrtić, a jedno u školu.

Because Croatian normally uses a collective numeral with children: dvoje, troje, četvero/petero, etc.

So troje djece is the natural way to say three children. This pattern is especially common with:

• children and young beings
• mixed-gender groups
• groups where gender is not important or not specified

For an English speaker, it helps to think of troje as a special three used with children as a group.

After collective numerals like troje and oboje, the following noun is typically in the genitive plural.

So:

• djeca = nominative plural
• djece = genitive plural

That is why you get troje djece, not troje djeca.

The preposition iz takes the genitive case, because it means from / out of.

So:

• zgrada = nominative singular
• zgrade = genitive singular
• naše matches it, so it is also genitive singular feminine

That whole phrase means from our building, and grammatically it is iz + genitive.

Because the verb for children playing is igrati se.

In Croatian:

• igrati se = to play, have fun
• igrati without se usually means to play something, such as a game, sport, role, etc.

So here se is not optional; it is part of the verb meaning.

There are two things happening here.

First, troje djece is treated as a collective subject, so in the past tense Croatian often uses the neuter singular form:

• igralo

Second, with a reflexive verb like igrati se, the 3rd person singular perfect normally appears without je:

• Dijete se igralo
• Troje djece se igralo

So se igralo is the expected standard form here.

Oboje means both, but specifically in the collective pattern used for children or a pair considered as a group.

It matches the same type of counting as troje:

• troje djece = three children
• oboje mlađih = both of the younger ones

So oboje is the natural form when talking about two children in this kind of construction.

Because oboje is followed by the genitive plural, and mlađih is the genitive plural of mlađi (younger).

Also, the adjective is being used substantively, meaning the noun is omitted because it is obvious from context.

So oboje mlađih literally means:

• both of the younger [children]

In this sentence, oboje is treated as a collective unit, so the verb is singular:

• oboje mlađih ide

That is a normal Croatian pattern with collective numerals. In English we say both go, but Croatian can view the pair as one grouped subject and use singular agreement.

Because Croatian often leaves out words that are easy to understand from context.

Here, the full meaning is:

• a jedno ide u školu

The verb ide is simply omitted because it was already used in the previous part. This kind of ellipsis is very common and natural.

Because the implied noun is dijete, and dijete is grammatically neuter.

So:

• jedan = masculine
• jedna = feminine
• jedno = neuter

Even if the child is a boy or a girl in real life, when the omitted noun is dijete, the form is still jedno.

Because the verb ići expresses movement toward a destination, and with that meaning u takes the accusative.

So:

• ići u vrtić
• ići u školu

If you wanted to talk about location instead, you would use the locative:

• u vrtiću = in kindergarten
• u školi = at/in school

Here the meaning is go to kindergarten / go to school, so the accusative is correct.

The semicolon separates two closely related ideas:

• first, what the children did today
• second, extra information about them

It works much like in English: stronger than a comma, but still linking two connected parts of one sentence.

The conjunction a then adds a mild contrast or shift:

• the two younger ones go to kindergarten, and the other one goes to school