Piano is always the best starter because like others said it is a great foundation to really learn the musical scale and be able to visualize the chord progression. I started with piano in 1st grade did violin for 1 year then started the drums in 4th. The piano background gave me a leg up in band class as I was one step ahead of the others in my experience and it helped me though out my music progression. Everyone loves a Piano player while the violin can make them run for the exits in the wrong hands.
The flute is ok when she gets older but I say a piano all the way.