I have really worked on my grandkids on this. Each has a whistle which they absolutely must have around their necks any time they are outside the tent. Before the trip, we practice the 3 toots ("Help, Come Quick") and they are warned never to use the whistle unless it's an emergency. They take this pretty seriously!

Be sure the whistle is the loudest you can find! A lot of so-called "survival" whistles can't be heard more than about 100 yards away!