Rhythm
Rhythm is an important part of music. Play a song or sing a song and have your child move to the beat. Then have them clap along. Pick songs at different tempos so that they learn that there are many different rhythmic speeds.​
Pitch/Notes
Music is made up of notes. Have your child listen to music and try to sing along. A good introduction to the notes of a staff is the song Do-Re-Mi, from the Sound of Music.
Expression
Discuss with your child the ways that they can express themself with their voice. They can be loud or soft just like this example with Kermit the Frog and Elmo. Music can be loud or soft too, it's called Dynamics. It is used to express emotions in music. In the Grizzly Bear Song pay attention to how the dynamics affect the feelings in the song. Have your kid pick a song they like and ask them to sing it really quiet, then moderately loud. Ask them how it changes how the song feels. Discuss other songs and how being loud or quiet can change how things feel.
Instruments
Discuss instrument sounds with your child. This is a video with many common instruments and their sounds. Encourage your child to describe the sounds they hear and copy them. If you have shakers or drums at home, show them the instruments and reinforce the sounds they make with hands-on demonstrations.