Had you said that to my wife, as someone who she just met, it would depend on her mood.  If in a good mood, she would probably just walk away from you .  If in a bad mood, you would get an earful about being a jerk.  My reaction would entirely depend on how she took it, but I can pretty much guarantee that it would be less nice than her reaction.  I don't have kids, but my friends who have daughters would probably react somewhat less politely than my wife. 

All that being said, had a friend or acquaintance from the gym paid her a compliment on her hard work, that would be a whole different story.   Everything has to do with the relationship you have with the person.