[blindkid] Child Finder-safety issues

[Marcia Headley]

Hi! I would like to know if any of you can recommend a device that can be used to detect when your child has gone too far away from your "field of vision"  in a public place. This is actually for my sighted son. He is four yrs old. I also have a 7 yr old daughter who is blind and we all go on outtings, my two kids, husband and I, I take care of my daughter and my husband watches my son. Last weekend we went to a kids museum and my son "lost his daddy" (it was all the way around) and it was very scary. This is the third incident and I am very concerned that our outtings are becoming more of a stressful experience than fun. I don't want my 4 yr old to feel responsible for being watching the adult in charge of his safety. I am just very concerned.
 
Do you know of something that the child can wear and the adult, at the other end, can wear as well so that if the child is out of range it buzzes or something. I was thinking about a walk-talk thing but my son is very young and I am afraid he might lose something like that. Maybe something he can wear around the wrist or something.
 
Thanks for your help.
Marcia-Mom to Lauren, 7yrs old (bilateral microphthalmia, blind and developmental delays) and Josh, 4yrs old (typical sighted boy)