I volunteered in Surin, Thailand, not in an orphanage but in a school. I was just out of school and ended up paying for the opportunity because my parents thought it might be safer.
When we got to the school we were given our classes but no guidance on how to teach them (we were encouraged to do a TEFL before coming which was included in the price of the trip but it wasn't mandatory). The organisers didn't seemed to care at all about the lessons we were putting across to the kids and the teachers just seemed to be happy that we were around them.
We quickly found out that everything we were trying to teach them had been taught by the people before us, and maybe those before them. So the lessons we had planned were a complete waste of time. We ended up doing a lot of music with them and rewriting words to popular songs so they could sing along and although I can't see that it was damaging in any way, it was definitely demoralising and frustrating for us and totally unnecessary for the children.
It also seemed to be an area that may have not needed volunteers, the school had many facilities, the children had uniforms, shoes, phones and iPads. Most were picked up from school by their parents. There were many teachers that seemed to be able to handle the class just fine. All we seemed to add was a native English speaker being around the kids.
Maybe this is enough, but it didn't feel quite right to me.