A chip being embedded in a child is far more dangerous than the risk of them being abducted by murderers/rapists/kidnappers/human trafficking employees. It's a foreign object, so there's a risk of the body rejecting it. And then there's a risk of infection from the surgery. The chip, being metal and submitting radio signals, would severely interfere with xrays. MRIs would be inaccessible unless they removed the chip.

I don't know about you, but I have two unruly boys who are much more likely to get a broken arm or concussion than be abducted by someone.

Not to mention those murderers/rapists/kidnappers/human trafficking employees will take that chip out anyway.

The only real use for it is if they pull a Stand by Me and go searching for a dead body to poke. I did that with my friends as a child, and I always came back.