At music time, I will communicate to the children that they need to either stop talking, singing, playing instruments, or stop dancing when I say "READY REST."  Sometimes, I will hold up a small sign with the rest symbol on it.  This will help them visually understand that it is time to 'rest.'  The rest sign is printed on a card in the shape of a stop sign which also helps to emphasize that they need to stop what they are doing. 

In music, this symbol is called a 'quarter rest' although in our music class, we simply call it a 'rest'...or it is sometimes referred to as 'ready rest'.