After some brief but intense research, I found four possible
candidates in my collections. All produced clearly audible tones
but only one would be a likely find for a child at a beach.
 
The top choice is Nassarius papillosus, the pimpled basket shell.
It could be forced to emit a loud tone that could be heard quite a
distance. It was the easiest to use, also.
 
Other 'whistlers' were Cymatium clandestinum, Colubraria maculosa
and, with difficulty, Polinices opacus. The Colubraria and the
Nassarius are reasonable sizes -- about 1-1/2 inches -- and could be
fixed with a string to hang around the neck.
 
Hope that gets you somewhere. If not, try what my wife mentioned
when I explained what I was doing with the shells... if you are
creating an island that doesn't exist, create a shell that whistles
just right. It will be easier to find.
 
Aloha,
 
makuabob (a.k.a. Bob Dayle)