Music Express Magazine produced a series called The Music Show. It does a great job of explaining musical terms using characters and in a way that kids can understand.  Here is the second episode explaining rhythm.

Rhythm is the length the notes are played.  There can also be times of silence in music call rests.  Episode Three of  " The Music Show" by Music Express Magazine explains how rests work in music. 

This is an example of the syllables we use to express rhythm.  We start with Ta and Ti-Ti then move on to more complicated rhythms.