A child that is equipped with both a whistle and the proper training on how to use it can be kept safe from danger whether in the city or wilderness.
There are couple distinct vintages to using a whistle.
First, you can only yell for safety as long as your voice holds up. Second, a human voice cannot carry as far or stand out as much as the piercing sound of a whistle.
All in all, attaching a whistle to your child is an excellent practice that could save his or her life.