Any one has volunteered in orphanages ? Any idea about the formalities here?

I guess it pretty much depends on which Orphanage you go. I used to volunteer at Pertapis and I would address the adults there as "Kak" (big sister in malay) unless they had introduced themselves with a salutation. The kids there call me "Kakak" or "Kak" and I honestly much prefer it that way because it's easier to gel with the kids that way and makes talking much more comfortable.