I'm a random member of the public who pays their taxes and has no criminal past. You seem to think that because a child is missing anyone accused of having that child should let their child be DNA tested for the sake of the missing child? How does that help the missing child? I am willing to demonstrate that my child is my child by normal means. I see no need for DNA testing and I very much doubt the police would either.
Your micro chipped much loved moggie which has a very distinguishing feature vanishes from your garden.
Six months down the line you spot a slightly fatter cat looking very much like yours with exactly the same distinguishing feature sitting in another person's window.
When you knock at the door you are given the assurance that the cat has lived there for the past four years. Would that be sufficient for you or would you prefer to check what the micro chip might say.
I think you fail to recognise that your individual preferences would not come into play as far as the police are concerned if there was the slightest suspicion that the child you claimed as your own might actually be a missing person ... you would have no choice in the matter ... and the majority of innocent parents would find it no big deal.