Another minority here...
Both of our cats are chipped and inside only animals. Yes, they also wear collars, but have gotten out of them on many occasions. I don't see anything wrong with it. The chip doesn't "transmit" anything. It has to be scanned with a device and we couldn't pin point our cat on a GPS system or anything like that. It doesn't bother them, it is just right under the skin.
That said, the chips are easy to put in and take out. I would definately put one in my children when they are young... especially if we are going to Mexico to visit my DH's family a lot. He is a "white" mexican and I am so white that our children are going to be extremely fair skinned... the type of children who get kidnapped. (He doesn't even want to take me because he is worried about me getting kidnapped). When they are older, it is their choice to have them taken out... with one stitch to close the tiny insision.
In those situations I can understand it. When there is a risk, we take action to prevent that risk. If you must, flame... but I probably won't be looking at this post again.