When I'm talking about their teachers with my kids, I call them Ms. Firstname (daycare) or Ms. Lastname (kindergarten). But what do you call the teacher when you're addressing them one on one?

I've always called the daycare teachers by their first names, but it seems more appropriate since they go by Ms. Firstname with the kids. I was just writing an email to E's kindergarten teacher though and seriously struggled about whether I should use her first name or call her Ms. Lastname. I totally avoided by just saying "hello!"