Voice lessons are a perfect supplement to any piano lesson (but I also offer voice training alone):
Classical voice training that teaches proper projection and pitch control
Use of the diaphragm and overall core strengthening
Breathing exercises and posture setting that benefits overall health
Solfeggio and sight-reading, ear training and tonal recognition
Most importantly however, singing is a wonderful way to enjoy "playing" music that does not require as much concentration or practice!
Children love to sing songs and it is a way to enjoy creating music together.
I play full accompaniments to our singing a variety of pieces from Disney to folk songs.
Siblings that are not part of the piano lessons are free to join in the singing portion if they are inclined to.