Familiar form of address for a young, subordinate, woman that connotes respect (Female equivalent to 'Son')

Use her name - in the UK, there is no expression you could possibly use that would not be considered insultingly paternalistic (if used by a man), over familiar, patronising or chauvinistic. In previous times, terms such as 'dear' 'love' 'duckie' 'sweetie' 'sugar' and even darling might have been used - none would be considered acceptable today in the circumstances you describe.

If you are looking for an informal word, then lass could work. But it doesn't carry the same information as to the relationship between the two people.

The gender-neutral "kid" or "child" comes to mind.

"Come here kid, I have something to tell you about rude customers..."

"Come here child, I have something to tell you about rude customers..."

The connotation might not be quite as familial as "son", but then again, I wouldn't really want anybody calling me "son" to start with... Of course, this also depends entirely on who is doing the speaking: both of the above sentences (and the original "son" sentence) could be construed as sounding negative coming from the wrong type of person.

The "correct" answer in this case is probably just to use her name.