So, wait... at this age, your daughter is already a shameless hussar?
Congratulations!  (And never ever tell her why that was funny). 
Foam sword - probably a good idea.  Fencing lessons - Probably not before third grade at the earliest. 
But she's a kid.  By Third Grade she'll be on Dinosaurs, collecting bugs, fairy tales, and some kid-catering singer who hasn't even started to get popular yet.  After going through sixteen other things on the way.  
Kids are little maniacs, and I mean that in the absolute best possible way.  It is entirely normal for kids to learn by constantly devoting effort to a constantly shifting set of manias. They do deep-dive learning on whatever the subject of their most recent obsession, and then after a while the obsession changes and she goes from, I dunno, a mania for dance lessons to a mania for track and field events.