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.