My two cents worth - If I am out searching and I hear someone tooting on a whistle, I would make tracks to check out the situation. I wouldn't worry about one or two whistles and their different meanings. Three signals, or three of anything is generally a signal for help, but even that can't be taken too literally. Could you imagine thinking - "they only blew their whistle twice - they must be fine, so we won't check them out."

The important thing is your kids, if they are ever lost or in trouble, should do everything to make themselves as conspicuous as possible (short of setting a forest fire) and attract attention. Don't sweat the details....