I agree with Tony Phạm. You should call they is "em" :) And they'll call you "anh" (because you are man, for woman, they'll call "chị").
However, if they look like too young and hesitate. I'm sure they'll call you by "chú" and call themself by "cháu".
Let's keep in touch and learn language together ;) :D my skype :
[email protected] , make a video call for me if you want, I would be happy for teaching Vietnamese and learning English from you :)