Tell a VERY animated story. Children will start singing too and it gets quieter. You can even sing quieter as more join you. Some people have a signal like a hand in the air or turn off the lights. A whistle might be used to signal quiet depending on your program.
Some people speak in a quiet voice.
The children want to know what is being said, so they get quiet. My children always sing with me. Try to redirect the child to an acceptable behavior. Praise a child who is doing the desirable behavior. Most others will follow before long.