"Shhhhhh!" That doesn't work well with kids, so here are some attention grabbers that will engage children in a positive way.
https://www.youtube.com/watch?v=vK6EKSUJ6Wg
Hint! The best attention grabbers involve children's hands as well as their eyes and ears. Keep doing the motions and chanting the words until the entire class is following along.
MORE? Here's another video with a few more tricks to help children focus.
https://www.youtube.com/watch?v=8xRVtrnnw-w&t=1s