Some kindergarten teachers might play the piano, or guitar, and provide music for the kids to sing along to. Not all of them will throw a CD on and play music through a sound system.
If the sheet music was legally purchased, then there's no reason the teacher can't play the song on the piano in the classroom. Usually, when such sheet music is purchased, the price includes such a license. I remember from grade- and high-school band that the charts we used were specifically licensed for our use (including performance).
I suspect the real issue is that the sheet music is being photocopied and handed out -- and that's the copyright violation.