1) The hidden kiss is a metaphor from Peter Pan. In short, it may represent preserved innocence- the part of oneself that remains forever youthful. Mrs. Darling in Peter Pan, keeps her hidden kiss to herself; not even sharing it with Mr. Darling. One's hidden kiss, reveals the truth of his or her innermost self; his or her true cares and beliefs; what makes a person tick.
2) In Peter Pan, the hidden kiss is located in the corner of the mouth, where the lip begins to turn out. It could just be a term for that area of the mouth. Or, maybe Mrs. Darling's "hidden kiss" is there, because that is the area where her facial expression changes… ruining her façade.
-If anyone is especially interested in this term, check out literary discussions that compare the metaphors of Mrs. Darlings hidden kiss, with Peter Pan and Wendy. Very poetical stuff!
Mrs. Darling's hidden kiss in J.M. Barrie's "Peter Pan."