A dog's teeth can be safely brushed with a paste of baking soda and a little water. However, it's a good idea to get it used to the idea of brushing first.
Start by massaging the dog's lips once or twice a day, then its teeth and gums. When it's used to this, put a little of the baking soda paste on its lips during the massage. When it seems comfortable with the baking soda, use a doggie toothbrush with soft bristles and just a little paste. Gently brush its teeth, one small section at a time, in a circular motion. If the dog fights you, back off to an earlier step, and try again later. When it's comfortable, start brushing its teeth regularly two or three times a week.