sono kodomotati ha kouen de buranko ni notte imasu.

A natural reading is:

その こどもたち は こうえん で ブランコ に のって います。

In normal writing, it would usually appear without spaces:

その子供たちは公園でブランコに乗っています。

その means that or those, depending on the noun that follows.

So:

• その子供 = that child
• その子供たち = those children

In context, その often points to something already mentioned, or something near the listener. In English, it may sometimes sound more natural as the, even though the Japanese literally has that/those.

たち is a common plural-like suffix used for people.

• 子供 = child / children
• 子供たち = children

Japanese nouns usually do not have to show singular vs. plural, so 子供 alone can already mean child or children depending on context. Adding たち makes it clearer that we are talking about more than one child.

So 子供たち is used here to explicitly show children.

は marks the topic of the sentence.

So:

その子供たちは = As for those children, ...

This does not always match the English subject exactly, but here it works very similarly. The sentence is talking about those children, and then says what they are doing.

A natural breakdown is:

• その子供たちは = as for those children
• 公園で = in the park
• ブランコに乗っています = are riding on the swing(s)

Here, で marks the place where an action happens.

• 公園で = in/at the park (where the action takes place)

Since riding is an action, the place where it happens is marked with で.

Compare:

• 公園で遊ぶ = play at the park
• 公園で走る = run in the park
• 公園でブランコに乗る = ride a swing in the park

By contrast, に is often used for destination, existence, or certain verb patterns, not for the general location of an action in this sentence.

Because the verb 乗る is used with に for the thing you ride on / get on.

Examples:

• 電車に乗る = ride the train / get on the train
• 自転車に乗る = ride a bicycle
• 馬に乗る = ride a horse
• ブランコに乗る = ride on a swing

So ブランコに is not marking location here. It is marking the thing being ridden.

This is an important verb pattern:

乗る + に

乗っています is the -ている form of 乗る.

• 乗る = to ride / to get on
• 乗っている = be riding / be on
• 乗っています = polite form of 乗っている

In this sentence, 乗っています shows an ongoing situation: the children are currently riding the swing(s).

By contrast:

• 乗ります can mean will ride, ride, or habitually ride, depending on context
• 乗っています clearly focuses on the ongoing state/action

So if the meaning shown to the learner is something like The children are riding on the swings in the park, then 乗っています is exactly what you would expect.

With 乗る, 〜ている often describes the resulting state of being on something, and in many contexts it also naturally corresponds to English is riding.

For example:

• 電車に乗っている = is on the train / is riding the train
• 自転車に乗っている = is riding a bicycle
• ブランコに乗っている = is on a swing / is riding a swing

So in this sentence, it gives the idea that the children are currently in the situation of riding the swings.

This is very common in Japanese: 〜ている does not always match English progressive exactly, but here are riding is a very natural translation.

Japanese nouns usually do not mark singular vs. plural.

So ブランコ could mean:

• a swing
• the swing
• swings

The exact number is understood from context.

Because the sentence has 子供たち (children), English might naturally translate this as the swings, but the Japanese itself does not force that plural meaning.

Yes. It is in the polite style because of います.

• casual: その子供たちは公園でブランコに乗っている。
• polite: その子供たちは公園でブランコに乗っています。

The polite style is very common in textbooks and general conversation when speaking respectfully or neutrally.

The spaces are probably there for learners.

Normally, Japanese is written without spaces:

その子供たちは公園でブランコに乗っています。

Textbooks and beginner materials often insert spaces to make the structure easier to see. This is useful for study, but it is not how standard Japanese is usually written.

Yes:

• その = those / that
• 子供たち = children
• は = topic marker
• 公園 = park
• で = in/at (place where action happens)
• ブランコ = swing
• に = on / onto, used with 乗る
• 乗って = te-form of 乗る
• います = polite いる, making 〜ています

So the structure is:

[topic] は [place] で [thing ridden] に 乗っています

In this sentence:

その子供たちは / 公園で / ブランコに / 乗っています。

Yes, が is possible, but it changes the nuance.

• その子供たちは公園でブランコに乗っています。
  = As for those children, they are riding on the swings in the park.

• その子供たちが公園でブランコに乗っています。
  = Those children are the ones riding on the swings in the park.

Very roughly:

• は presents those children as the topic
• が identifies those children more specifically, often with a sense of focus

In many basic contexts, both may translate similarly into English, but the nuance is different.