Hello classmates!
Here are my first sentences... some body can help me to check them:

Çocular bahçede oynuyor
-The children are playin in garden.

Okulda öğrencıler ve oğretmenler var
- There are students and teachers in the school.

Ben Ofısde çalışıyorum
- I-m working in the office

Gökyüzüde yildizlar parla
- The stars shine on the sky

Pazaryeride meyveler var
- There are fruits on the marketplace.

Tesekkurler

Greetings,
"Çocuklar" in that sentence is correct.

The accusative is to be added (to the object) when there is a transitive verb that takes the -i case.

In that sentence above çocuklar is the subject of the sentence and oynuyor is not a transitive verb and there is obviously no object since intransitive verbs don't take objects.

The story would be different here:

"Ben çocukları çok severim".

The reason is that it is "-i sevmek".

Another example:

"Çocukları gördün mü?"

Again it is çocukları because it is "-i görmek".

About consonant harmony the rules is this:
The consonants "p,ç,t,k,h,s,ş,f," are followed by their own kind if there is another consonant next to them.

Therefore we say okulda, İstanbul'da, Türkiye'de but we say Irak'ta, Paris'te, ofiste.

Yes, caliptrix is right. The sentences he suggests are possible. Of course this is another issue.

If we check caliptrix's sentences we see that interestingly there are two different -i suffixes.

This one is the accusative case:

"Çocukları çok severim."
It is because the verb sevmek is transitive and takes accusative case as "-i sevmek".

This -i is a possessive suffix:
"Onların çocukları bahçede oyun oynuyorlar."

"onların çocukları" has the same structure as "benim arabam", "senin telefon numaran" etc. There is a possessive adjective in front (benim, senin, onun, onların) and a noun with a possessive suffix afterwards (çocukları, arabam, numaran etc).