No no no no no!
(although pmsl at the mooncup idea )
For those of you making the pets/children comparison, I give you this story:
Friend of mine buys horse. Arab stallion, beautiful thing, v. expensive to insure. Insurers say cost of insurance will plummet if horsey is microchipped.
Geegee duly gets microchipped (and a natty little freezemark on his shoulder to prove he's been chipped). Four years later, friend decides to sell. As part of the 'yes I really do own this horse and am entitled to sell him' process, the vet comes along with his scanner and runs it over the horse's neck to confirm the microchipped horse is who he says he is (iyswim).
No microchip. Vet runs scanner over entire horse, several times as 'this is impossible', still no microchip. It's either become lodged somewhere between a couple of internal organs that are blocking the signal, or the horse has somehow passed it out of him. Either way, it's moved and, according to the vet, probably entered the bloodstream.
Would you still want to do this to your child? Even if the possibility was on a par with your child getting abducted (ie minimal)?
Just a thought....