Gritting or grinding teeth (the medical term for it is bruxism) is fairly common among young children, affecting three out of every ten. It is seen most often among kids under age five. No one knows exactly why they develop this tendency, but it could be because their upper and lower teeth aren't properly aligned. Other possible causes are pain – such as an earache or eruption of a tooth – or even stress, perhaps due to frustration or tension at home or some change in routine.
Most kids outgrow this habit when they lose their baby teeth and, fortunately, it usually doesn't hurt their permanent ones. As long as your granddaughter doesn't complain of soreness in her jaw or face in the morning, she's unlikely to suffer any ill effects. However, to be sure that the grinding isn't wearing down the enamel on her teeth, it's a good idea to take her to a pediatric dentist, who should be able to figure out whether the grinding is due to misaligned teeth or something else.
If the problem is stress, reassurance may be all that's needed once you know what the trouble is. Getting into a routine of relaxation before bedtime can help, as well– a warm bath, music, or reading a book, for example. If a child's teeth are being damaged by the grinding or when kids complain of pain or soreness in the face or jaw, a dental appliance called a night guard should be considered. Your dentist can tell you whether this is necessary, but very young children generally don't need guards. (If one is recommended, you're probably better off with a guard molded and fitted by the dentist rather than one sold over the counter, which may not fit or work as well as a custom appliance.)
I wouldn't worry too much about this. In all likelihood, your granddaughter will outgrow the grinding by the time her permanent teeth come in.
Andrew Weil, M.D.