A simple pealess signal whistle for a pack strap, a zipper pull or a child's jacket.

A shout carries a few hundred feet and your voice is gone in twenty minutes. A whistle carries much further, costs you almost no breath, and works just as well on hour six as hour one. That is the entire argument for carrying one.

The recognised distress signal is three blasts, repeated. Three of anything — whistle blasts, flashes, fires — means help. One blast means I am here.

Put one on every pack and every kid in the group. At three dollars there is no reason not to.