I found your question interesting, because I could not immediately remember what the little girl on the Ed Sullivan show was playing. I went back and checked the text.
". . . a frenzied little piano piece with a mesmerizing quality, which alternated between quick, playful passages and teasing, lilting ones. . . It was being pounded out by a little Chinese girl, about nine years old, with a Peter Pan haircut."
The little Chinese girl on the Ed Sullivan show is for sure playing a piano in Amy Tan's "Two Kinds."
It doesn't really make a difference though. The point of the story would be the same even if the little girl was playing a violin (or guitar, etc.). Suyuan was inspired by the little girl's ability so much that Suyuan continued to forcefully push Jing-Mei to do exactly the same. In this case, play the piano. Suyuan enrolled her daughter into piano lessons and made her practice all of the time. The only thing that this accomplished though was to drive a bigger and bigger wedge between mother and daughter. The entire sequence ends in a shouting match of hurt feelings.
"Then I wish I weren't your daughter, I wish you weren't my mother," I shouted."
Their relationship would have been driven to this point regardless of the instrument.