[blparent] child locators?

[Pipi]

Hey y'all,
A friend of mine is wanting information on child locators that she can use 
for her twins. She is wanting something reliable, that someone actually 
knows works. She is wanting something small that clips on. She needs it for 
when her kids are at a play gym, where they aren't wearing shoes.
If anyone uses one that is good, can you please let me know the name and 
possibly where it can be purchased.
Thank you,
pipi