I think that every child (and adult for that matter) should have some type of cell phone. Cell phones have helped out in so many kidnapping cases, for both children and adults.
Make them use a password. Create a password that you know and give to them and tell them if someone comes to pick them up, they must have the password. If they don't, tell them to NOT go anywhere with them.
Most kidnapping victims are kidnapped by someone they know ....
_________________________
A mistake is only a mistake if we fail to learn from it