I have just been thinking of this myself...not for my youngest DS (like you would assume) but for the
13 yr old!! 
He has ADD is get distracted very easy. It happened once at Sea World in San Antonio when he was 8, and was a horrible experience! He still remembers asking a lady to help him find his parents and all she said to him (an EIGHT yr old) was "I don't know your parents" and left him!

I was thinking of giving him my mom's cell to put in his pocket, for "emergencies" only, then he could call me on mine.