As crianças brincam no recreio da escola.

As is the feminine plural definite article, meaning the.

So as crianças = the children.

Portuguese often uses articles in places where English may be more flexible. In this sentence, as crianças sounds natural and complete.

Because criança is a grammatically feminine noun.

That does not mean it refers only to girls. It can refer to any child, male or female. So you say:

• a criança = the child
• as crianças = the children

Even if the group includes boys, the noun still stays feminine because that is the grammatical gender of criança.

Brincam is the 3rd person plural form of the verb brincar.

It matches the subject as crianças, which is plural.

Basic present-tense forms of brincar:

• eu brinco
• tu brincas
• ele/ela brinca
• nós brincamos
• eles/elas brincam

So:

• As crianças brincam = The children play / are playing

It can mean both, depending on context.

In Portuguese, the simple present often covers:

• a general or habitual action: The children play
• an action happening now: The children are playing

If you want to make the ongoing action very explicit in European Portuguese, you often use estar a + infinitive:

• As crianças estão a brincar no recreio da escola.

Because brincar and jogar are not used in exactly the same way.

• brincar = to play in a general sense, especially for children playing, having fun, messing around
• jogar = to play a game, a sport, or something with rules

So in a playground or recess context, brincam is the natural choice.

Examples:

• As crianças brincam no recreio. = The children play in the playground / at recess.
• As crianças jogam futebol. = The children play football.

No is a contraction of em + o.

• em = in / on / at
• o = the

So:

• no = in the / at the

Here, no recreio means in the playground or at recess, depending on context.

Because recreio is masculine singular, you use o, and that gives no.

Compare:

• no = em + o
• na = em + a

In a school context, recreio can refer to:

• recess / break time
• the playground / schoolyard area

So no recreio da escola can suggest the place, the time, or both, depending on context.

That is why learners sometimes see it translated in slightly different ways.

Da is a contraction of de + a.

• de = of / from
• a = the

So:

• da escola = of the school

Portuguese often uses this structure where English prefers a noun + noun structure.

So:

• o recreio da escola literally = the playground of the school
• natural English = the school playground

Because escola normally takes the definite article here: a escola.

When de comes before a, they contract:

• de + a escola → da escola

In this sentence, leaving out the article would sound unnatural.

So:

• natural: o recreio da escola
• not natural here: o recreio de escola

Yes, to some extent.

The sentence as given is the most neutral order:

• As crianças brincam no recreio da escola.

But Portuguese can move elements for emphasis:

• No recreio da escola, as crianças brincam.

That sounds more like In the school playground, the children play.

Portuguese can also omit the subject if it is clear from context:

• Brincam no recreio da escola.

But as a standalone sentence, keeping as crianças is clearer and more natural for a learner.

Yes, it works perfectly in European Portuguese, and it is also understandable in Brazilian Portuguese.

One important variety difference appears when you want to emphasize an action happening right now:

• European Portuguese: As crianças estão a brincar no recreio da escola.
• Brazilian Portuguese: As crianças estão brincando no recreio da escola.

So the sentence you were given is fine in Portugal, and its grammar fits European Portuguese well.