Sharps In Brackets

9-yr-old girl has just started learning a jazzy piece with lots of 'blue' notes.  It's in the key of A major (F#, C#, G#).

Me [pointing to the first C# in the piece]: Is this a high 2, or a low 2?
Girl [promptly]: High.  Because it's not got a natural, and it's a C, and there's a C# in the key signature.
Me [pointing to the C in the second bar]:  How about this one?
Girl [promptly]: Low. 
Me: Because?
Girl [like she's talking to someone a bit slow]: Because there's a natural in front of it, and that takes away the sharp.
Me: Good.  How about this second finger?
Girl: Umm..  High.  I think.  Why is the sharp in brackets?
Me: What do you think that means?
Girl: It's optional?
Me: Er- no.  Although I like the idea. But, can you imagine if you had a whole orchestra doing optional sharps?  [girl starts giggling] It would be madness! 
Girl [still giggling]: It would sound - blergh...
Me: Exactly.  So they're actually trying to be nice to you, because in the bar beforehand that note was natural. 
Girl: Oh.  And is that the same here and - wow.  They're everywhere!