Toddlers and Manners

My daughters are becoming pretty good at saying "please," "thank you," and "you're welcome." Sometimes they need a prompt or two, but overall they're catching on. There's one aspect of manners that really has me stumped: how should children address adults?
Growing up I was always taught to call adults "Mr. X" or "Mrs. X." I remember one friend's mom told me it was OK to call her by her first name, but my parents still insisted I address her as "Mrs. X." I think I was out of college before I addressed a long-time older neighbor and friend by her first name. These days, however, I seem to only hear children call adults by their first name.
I don't know if it's necessarily rude. It doesn't really bother me when friends' children call me by my first name. I used my maiden name professionally even after I was married, so being referred to by my married name sometimes throws me for a loop. But how do you explain to children that they need to call their teachers "Mr. or Mrs." but other adults can be addressed by their first names?
If it's someone I don't know very well I will introduce the person to my children as "Mrs. X." When it comes to my friends, though, it's a different story. I'm never going to remember not to call my friends by their first names when talking to my daughters, so how can I expect them to call them anything else?
I know there are some adults who will correct children who call them by their first names. Is it a sign of respect? Absolutely. Do I think children who address me by my first name are disrespectful? No, not really. I guess I'm writing this post for input. How do you ask your children to address adults? Is it a big deal to call them by their first name?