Rather infamous story at bmi of SMB telling off one of our ground staff for greeting him with "Hi there," responding "Shouldn't that be good morning, sir?" Totally depends on the customer, or group of customers, you're speaking to and the situation, I don't think there's a blanket rule. Personally I prefer "Ladies and Gentleman, Boys and Girls" for announcements, "Sir or Madam" for initial interaction and to continue with unless told the customer isn't comfortable with being told this (happens more than you'd think, will never forget a thick-Brummy-accented pax on a flight from BHX saying "no airs and graces here bab, we all sh*t and p*ss out the same hole") Usually call the children "young man or young lady" unless their parents introduce themselves.