I've tried a number of whistles, and the ones my family and I always carry are these from REI
http://www.rei.com/product/726607
http://www.rei.com/product/726608
They are plenty loud and relatively easy to get some good sound out of, plus they meet your criteria of being easy to carry with dog tags. Another good point is that it should be pretty easy to get a bunch of them from an outfit the size of REI.
http://www.rei.com/product/726607
http://www.rei.com/product/726608
They are plenty loud and relatively easy to get some good sound out of, plus they meet your criteria of being easy to carry with dog tags. Another good point is that it should be pretty easy to get a bunch of them from an outfit the size of REI.