Our expert says:
Folklore usually sites iron deficiency or worms as the cause of teeth grinding but some children just do it and grows out of this stage with time. Nervous tension can be an underlying cause and it may help to let him take a relaxing bath and wind down towards bedtime.
See also http://www.preschoolerstoday.com/articles/health-issues/escaping-the-nightly-grind-3050/
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