It depends on where you are. People who let their kids cry/scream in a restaurants /movies need to rethink what they are doing. Have a plan and accept that you may need to get up and walk out for a few minutes. On a plane or bus or DMV, there just isn't a lot you can do.
http://www.dudeiwantthat.com/omg/fools/the-baby-muzzle.asp