I was /am following this thread because I am on the same path to bring my family. According to 2nd comment we just need a passport, nothing else?
I know a KTM University lecturer , wanted to participate in IEEE conference. He went to American embassy with a IEEE participation letter and passport. You can guess what was the outcome, do I have to mention it?
At least you should have the documents mentioned by Taremam.

I am not from a rich family, my parents were not able to sent all the children to a privet school for that reason I prefer to write in native language. Though it does not go smooth as intended.

Something stocked in my head about the line "remember, my parents walking in-out just with
passports in hand". Am I correct, these two fellows are well known to each other and reminding him how his/her father got US visa? I am lost in translation.

Before commenting, I googled and found this explanation.
"When you're asked to remember something it means that you are being requested to retain and recall information or an experience at a later time."