I don't think you'd be a buttinsky...not knowing the parents "at all" isn't good enough. The parent/s trusted you enough to be in charge of their teenage daughter at an amusement park...you would simply be expressing concern over something you noted...treat it no differently than you would noticing her smelling like nicotine after a bathroom run. Your medical backround gives you insight to things others may overlook...worst case the parent feels ignorant for not having noticed something about her own child (but shouldn't unless they share the same medical knowledge as you and even then). If this or another parent noticed something about one of your children that you may or may not know how would you feel about them checking in with you?
Do what you feel is in the best interest of the child and you've got nothing to loose.
