I'll endorse the whistle idea. I "bell" all my kids when I take them out into the woods, and tell them what they are supposed to do if they mess up and get seperated (hug a tree, blow three times, wait, listen, blow three times).

Hey, question for all the SAR bubbas as I drift a little here - is it one whistle to say "I hear you, stay where you are, I'm coming to you" and two whistle blasts to say "come to me" - or do I have it reversed? I want to make sure I'm teaching them right up front to save issues later.