Когда дети играют в мяч во дворе, я сижу на скамейке.

Yes. In this sentence, Когда дети играют в мяч во дворе means When the children are playing / when the children play in the yard.

Here когда introduces a time clause, so the sentence has:

• a subordinate clause: Когда дети играют в мяч во дворе
• a main clause: я сижу на скамейке

Very often, когда works exactly like English when in sentences like this.

Also notice the comma:

• Russian normally uses a comma before or after a subordinate clause.
• Since the когда clause comes first, the comma appears before the main clause.

Because дети is plural, so the verb must also be plural.

• дети = children
• играют = they play / are playing

The verb играть in the present tense changes by person and number:

• я играю = I play
• ты играешь = you play
• он/она играет = he/she plays
• мы играем = we play
• вы играете = you play
• они играют = they play

Since дети is grammatically plural, Russian uses the same form as with они: играют.

Because Russian says играть во что? for playing a game or playing with a ball in this kind of expression.

So:

• играть в мяч = to play ball / to play with a ball
• играть в футбол = to play football
• играть в шахматы = to play chess

This is just the normal Russian pattern. English often says play ball or play football without a preposition, but Russian usually needs в here.

Also, мяч changes form:

• dictionary form: мяч
• after в in this pattern: в мяч

That is the accusative case.

Во is a pronunciation variant of в. It is used before some consonant combinations to make the phrase easier to say.

So:

• в мяч is easy enough to pronounce
• в дворе is awkward, so Russian uses во дворе

This does not change the meaning. В and во are the same preposition here.

You will see this in other common phrases too:

• во вторник = on Tuesday
• во Франции = in France

It is the prepositional case.

The phrase во дворе means in the yard / in the courtyard, and after в meaning location, Russian usually uses the prepositional case.

So:

• двор = yard, courtyard
• во дворе = in the yard

This is an important contrast:

• в + accusative often shows motion into somewhere
• в + prepositional often shows location in somewhere

Compare:

• Я иду во двор. = I am going into the yard.
• Я во дворе. = I am in the yard.

Because на скамейке describes location: on the bench.

After на, Russian often uses:

• accusative for motion onto something
• prepositional for location on something

Compare:

• Я сажусь на скамейку. = I sit down onto the bench.
• Я сижу на скамейке. = I am sitting on the bench.

So in your sentence:

• сижу = am sitting
• this is a state/location, not movement
• therefore на скамейке is prepositional

Сидеть is the infinitive, meaning to sit.
In the sentence, you need a conjugated form, because the subject is я.

So:

• сидеть = to sit
• я сижу = I sit / I am sitting

This verb is irregular in the present tense, so the form is not predictable from the infinitive in a simple way. You just learn it as:

• я сижу
• ты сидишь
• он/она сидит
• мы сидим
• вы сидите
• они сидят

It can mean either, depending on context.

Russian present tense often covers both:

• simple present
• present continuous

So:

• дети играют can mean the children play or the children are playing
• я сижу can mean I sit or I am sitting

In this sentence, the most natural English translation is probably:

• When the children are playing ball in the yard, I am sitting on the bench or
• When the children play ball in the yard, I sit on the bench

Russian itself does not mark that distinction the way English does.

Because the sentence describes an ongoing or habitual situation, not a completed action.

The verbs are:

• играть → играют
• сидеть → сижу

Both are imperfective, which is normal for:

• background actions
• repeated actions
• ongoing situations
• general habits

Using perfective verbs here would sound wrong, because the sentence is not about one completed event. It describes what is happening or what usually happens when the children are playing.

The word order can be changed, but the original version is the most neutral and natural.

Original:

• Когда дети играют в мяч во дворе, я сижу на скамейке.

You could also say:

• Я сижу на скамейке, когда дети играют в мяч во дворе.

Both are correct. The difference is mostly about emphasis and style:

• starting with Когда... emphasizes the time/background first
• starting with Я сижу... emphasizes I am sitting on the bench

Inside the clause, word order can sometimes move around too, but not all versions sound equally natural. For a learner, the original order is a very good model.

Russian often can omit subject pronouns, because the verb ending already shows the person.

So in some contexts, сижу на скамейке could already mean I am sitting on the bench.

However, я is often still included:

• for clarity
• for contrast
• for emphasis
• simply because it sounds natural in that sentence

Here я is completely normal. It helps clearly separate the main clause from the subordinate clause:

• Когда дети играют..., я сижу...

So yes, Russian can omit pronouns more often than English, but including я here is perfectly standard.

Во дворе most literally means in the yard or in the courtyard.

Depending on context, двор can refer to:

• the yard of a house
• a courtyard between buildings
• an outdoor area near a building

So in English, the best translation might be:

• in the yard
• in the courtyard
• sometimes more loosely outside in the yard

It does not just mean outside in a general sense. It refers to a specific outdoor area: the двор.

Usually скамейка means bench.

Most often it refers to:

• a park bench
• a simple bench for sitting
• a bench in a yard or outdoor area

In some contexts it can also refer to a narrow bench in a gym or other setting, but bench is the basic meaning.

So на скамейке here is very naturally on the bench.