Don't mention it if the VO doesn't ask.
2. What kind of material do I need to prepare for my parents applying the B-2 visa?
Invitation letter, copy of passport, I-94, job offer letter, pay check, bank statement for 6 months, and I-134
3. One friend said that I need to mail my ORIGINAL H-1B approval notice back to my parents, since they need it for their visa appointment. Is it correct?
No. Copy is ok.