it is clear ?

"ah.... so adorable... When big borther gone to kindergarten, little bro seems like that how express his sad !"

please fix my sentence and i wanna be nature and like talk to my friend. この表現は自然ですか？

Our "ah" is more like Awww. It sounds a lot like 어 or 응.

"Aww, so adorable. When his big brother goes to kindergarten, he/(name) expresses how sad he is about it."

We don't say "big brother" (형) and "little brother" (동생) in English at all. Unless we refer to someone else's brother, which would be the subject of the sentence.

So instead little brother's name.