Is 'black line' a danger to the child or his teeth?
It isn't harmful to the child or his teeth. It just needs to be cleaned off, by a professional probably. Then you should take extra care to brush the child's teeth a little longer each time you brush them and brush them at least twice a day. Don't brush harder or use a hard bristle brush. Use a soft bristle brush and brush longer. The child should eventually get rid of the bacteria in his mouth that is causing the staining. Try changing a few things in his diet; less sugar, less caffeine, stuff like that. If you talk with his dentist, they will have better suggestions for you also.