Nawet wtedy, gdy pada deszcz, dzieci bawią się na placu zabaw.

It means even when or more literally even then, when.

• nawet = even
• wtedy = then / at that time
• gdy = when

So the structure is a bit more expanded than English. Polish often uses wtedy, gdy... to mean when..., especially when it wants to sound a little more explicit or formal.

A simpler version would be:

Nawet gdy pada deszcz, dzieci bawią się na placu zabaw.

That means basically the same thing.

Yes, kiedy would also work.

• gdy = when
• kiedy = when

In many everyday contexts they are interchangeable. The difference is mostly stylistic:

• gdy can sound a bit more written, smooth, or formal
• kiedy can sound a bit more neutral/conversational

So:

• Nawet wtedy, gdy pada deszcz...
• Nawet wtedy, kiedy pada deszcz...

Both are correct.

Polish often expresses weather more literally than English.

• padać = to fall in the sense of rain, snow, etc.
• deszcz = rain

So pada deszcz literally means rain is falling, but in natural English we usually translate it as it is raining.

This pattern is very common:

• Pada deszcz. = It’s raining.
• Pada śnieg. = It’s snowing.

Polish does not need a dummy subject like English it.

Deszcz is in the nominative case.

It is the grammatical subject of pada.

• deszcz = nominative singular
• verb: pada = falls / is falling

So even though English says it is raining, Polish treats rain itself as the subject.

Dzieci is the nominative plural of dziecko (child), and it means children.

Here it is the subject of the sentence:

• dzieci = children
• bawią się = play / are playing / have fun

So:

dzieci bawią się = the children are playing

Because the verb here is bawić się, which means to play, to enjoy oneself, or to have fun.

This is important:

• bawić = to amuse someone / entertain someone
• bawić się = to play / have fun

So:

• Dzieci bawią się. = The children are playing / having fun.
• Rodzice bawią dzieci. = The parents entertain the children.

The się is part of the verb in this meaning, so you usually cannot leave it out.

In this sentence, się is part of the reflexive-style verb bawić się.

It does not always mean a fully literal oneself. Very often in Polish, verbs with się simply have their own meaning and must be learned as a unit.

So it is best to learn:

• bawić = to amuse / entertain
• bawić się = to play / have fun

Also note that się often comes after the verb in normal word order:

• Dzieci bawią się na placu zabaw.

But in some contexts its position can move:

• Dzieci się bawią na placu zabaw.

Both are possible, though the first is very natural here.

Both verbs are in the present tense and are imperfective.

• pada = present tense of padać
• bawią się = present tense of bawić się

Why imperfective? Because the sentence describes a general/habitual situation:

Even when it rains, the children play on the playground.

It is not about one completed event. It is about something that happens repeatedly or characteristically.

Here na means on / at, and because it refers to location, it takes the locative case.

So:

• plac zabaw = playground
• na placu zabaw = on/at the playground

The word plac changes to placu because it is in the locative singular after na.

A useful contrast:

• na placu zabaw = at the playground (location, locative)
• na plac zabaw = onto/to the playground (direction, accusative)

In this sentence the children are already there, so we use location: na placu zabaw.

Because plac zabaw is a fixed noun phrase made of:

• plac = yard / square / area
• zabaw = genitive plural of zabawa (play / game / amusement)

Literally, plac zabaw is something like a place/area for play.

When the whole phrase changes case, the main noun plac changes:

• plac zabaw → nominative
• na placu zabaw → locative

But zabaw stays in the genitive plural because it depends on plac as part of the compound expression.

Because gdy pada deszcz is a subordinate clause, and Polish normally separates subordinate clauses with commas.

So the structure is:

• main clause: dzieci bawią się na placu zabaw
• subordinate time clause: gdy pada deszcz

In Polish punctuation, this clause is set off by a comma:

Nawet wtedy, gdy pada deszcz, dzieci bawią się na placu zabaw.

This is standard and important in writing.

Yes. Polish word order is more flexible than English, because case endings carry much of the grammatical information.

For example, you could also say:

Dzieci bawią się na placu zabaw nawet wtedy, gdy pada deszcz.

This version is also correct. The meaning stays basically the same, but the emphasis changes:

• starting with Nawet wtedy... emphasizes the contrast: even then
• starting with Dzieci... emphasizes the children first

So the original sentence is natural, but not the only possible order.

Most naturally, it describes a general habit or typical behavior.

Because both verbs are in the present imperfective, the sentence usually means something like:

Even when it rains, children play on the playground.

That sounds habitual or generally true.

If you really wanted to stress that it is happening right now, context would usually make that clear. On its own, the sentence sounds more general.