The first thing i would address when discussing chips being implanted in children under five is what happens when they are older than five or even when they become adults? Do you then take the chip out? There comes a time in a persons life when they need to start making their own decisions and take responsibility for themselves, while at the same time parents need to start trusting their children to take care of themselves. Implanting a semi-permanent tracking chip into a child seems a little outrageous. I do however think that it would be a good idea to have chips built into shoes that would allow parents to know where their children are yet they have the ability to leave the chip at "home" if necessary like when a child gets to a certain age. The same idea goes for elderly people having the ability to track them with shoes but also knowing when not to track someone is important.
In general i think that implanting chips into a person to be able to track them pushes the limit of trust when it comes to teenagers and privacy and parents should respect that.