Great point, Jingo--and welcome to the forums! And the wilderness/survival forum, in particular!
You're right about whistles being much better for signaling for long periods than just the voice. Cody Lundin, in his excellent survival book
98.6 Degrees: The Art of Keeping your A__ Alive, advocates carrying a whistle on you at all times when outdoors.
More starkly, there was a case in Arizona some years back in which a kid got lost, was searched for, might have been found had he been able to make a loud enough noise, but was not found in time, and died. An auto service chain ended up responding to this event by starting a program of providing whistles (and encouragement to use them for survival purposes) free of charge to kids. Here is a link with the story, rationale, etc.:
http://www.equipped.org/kidwhstl.htm Kids' voices are surprisingly hard to hear sometimes, and I imagine it doesn't get better when they're scared speechless, exhausted, and hoarse from crying for a day straight.
One tricky thing is getting the kids to actually carry them--whistles tend to get played with, then left in the car while the kids go running off into the woods. Anticipate the need to push hard to get them to really take up the habit of carrying whistles wherever they go.
Another thing: different whistles take different amounts of air to make them work. I was surprised to find that the Storm whistle I'd bought for my tiny daughter was simply too big for her to make more than an almost-inaudible "toot". A Fox 40 Micro takes even more air--a lot of kids will probably be simply unable to use them. Good idea to test what you're thinking of using. Also, some people swear by "pea-less" whistles; it's said that the Fox 40 original was designed after a referee blew his whistle during a game, but got no sound out of it for some reason (e.g., pea frozen in place in cold weather, or something like that). So: not all created equal; have the user test it in the safety of home before relying on it to summon search & rescue.
Once more, welcome!