I was alone one day of last summer.

More natural to omit the of here I think.

I was walking around the beach and I saw 2two children near the river.

Child = 1 child, children is used for 2+

It's a little more natural to use words for small numbers in writing.

I have stopped in front of them and I have asked "who is the older guys of youasked "Which of you is older?"

"I have stopped in front of them" makes it sound like you're narrating this as you're standing there, having just stopped a few moments ago. Since you're talking about last summer, it's better to use the standard past tense.

The guy with the blonde hair has gigaved me an answer ,he said, "My name's jJhon and I'm the older guy thenone. That's my brother nearxt to me".

Same comment about the past tense.

The past tense forms of give are "gave" (standard) and "given" (used with has).

You don't need to say "he said", it's implied in "he gave me an answer"

It's "older than" when comparing. We also wouldn't really combine these two facts (that the other guy is his brother, and that he's older) into one clause in English.

People generally wouldn't refer to themselves as "guy" / "boy" / "girl" / similar words in sentences like this, so I changed it to "one"

Thank you for your quickly response Jhon.

"quickly" is the adverbial form. It's used for modifying verbs. "response" here is a noun, so you use the adjective form "quick" instead.

By the way, I'm assuming Jhon is a name from your country, but just in case it's meant to be the English name, the English name John is spelled as John or Jon.

After this I preferred not to continue the conversation and I started again to walking on the beach again.

One day ofin summer, while I was walking on the beach

I was alone one day of last summer.

I was walking around the beach, and I saw 2 child near the river.

I have stopped in front of them and I have asked "who is the older guys of youst?"

You could leave the I there but then you would need a comma after "them"

The guy with the blonde hair has gived me an answer ,he saidanswered me, "My name's jJhon and I'm the older guy then my brother near to me".

Thank you for your quickly response Jhon.

If you said this you would need quotes around this sentence

After this, I preferred not continue the conversation, and I started again to walking on the beach again.

There is a comma here because you have two independent clauses here.

I was alone one day of last summer.

I was walking around the beach and I saw 2two children near the river.

I haved stopped in front of them and I have asked "who is the older guys of you?"

"Which of you is oldest?" or "Which one of you is older?" would be a more appropriate phrasing.

The guy with the blonde hair has gigaved me an answer ,h. He said, "My name's jhoJohn and I'm the older guy then my brother near to me".

Thank you for your quickly response Jhohn.

After this I preferred to not continue the conversation and I started again to walking on the beach again.

I was alone one day ofwhole day in the last summer.

I was walking around at the beach and I saw 2 children which were near the river.

I have stopped in front of them and I have asked "who is the older guys ofIs there someone accompanying you?"

The guy with the blonde hair has gigaved me an answer ,he said, "My name's jhon and I'm the oir 'Older guy' also then my brother near to me".

After this I preferred to not continue with the conversation and I started again to walking on the beach.

I was walking around the beach and I saw 2 children near the river.

I have stopped in front of them and I have asked "who is the older guyst of you?"

The guy with the blonde hair has gigaved me an answer ,and he said, "My name's jhoJohn and I'm the older guy thean my brother near to mhere".

Thank you for your quickly response Jhohn.

After this I preferred to not continue the conversation and I started again to walking on the beach.