In certain circumstances I'd also be concerned that I was taking away a local teacher's job, though that's not the situation here.
On top of that, I'm not qualified and they are, with all the benefits that being qualified brings.
As I see it, the kids are best served if I help with spoken English in the presence of the regular teachers, and having two adults present will give more individual attention and help keep the children on task.
Both teachers returned to work last week. They told me they'd been ill, not taking time off because I was there. They understood the potential to take advantage of volunteers and my unease. These are two professional teachers and they offered to phone me when they were next off work. I know they understand, so when they're off again I'll probably go in anyway just to fill the gap for the kids.