Mr Happy: As a lefty, I can assure you that virtually all devices are designed for right-handed people. With musical instruments, the criterion is not which hand plays melody. It appears to me that the main consideration is which hand is called upon to do the more difficult task. It's possible that natural arm position (dominant hand closer to the body) may sometimes be a factor.

With piano, there tends to be more intricate finger movement for the right hand, particularly where the left hand is playing mainly chords. With guitar and banjo, the difficulty of learning fretting notwithstanding, the more demanding job is the picking, getting the aim and timing right (obviously this would not necessarily apply to those who only strum chords). Likewise, with fiddle/violin, the bowing would be considered the trickier skill to master.

Daddy Blues: It's going to get down to personal choice. I chose to play a lefty guitar because I have a strong sense of left-handedness in which holding a guitar in the right-handed position felt very awkward to me. The obvious disadvantage is the relative scarcity of instruments and the impossibility of swapping/sharing with other players.

The other options involve a righty guitar, which solves that problem. If your daughter feels quite comfortable holding one right-handed, that can work. The possible drawback, however, is that if she develops an interest in picking rather than just strumming, she may never realize her full potential with a righty guitar.

And then there's playing a righty guitar upside down, holding it as though it were left-handed. This entails working out unconventional fingering and usually "upside down" strumming/picking. Some have done this (Jimi Hendrix, Elizabeth Cotton); I haven't so I don't know what it's like.

Looking for a guitar at a store that carried only right-handed models, I was told that I should play right-handed so my left hand could do the fretting--the "hard part." Which raised the question in my mind, if that's such an advantage, then why aren't most guitars made left-handed so that the majority--right handers--could benefit from it. Food for thought.