Don't fall for the line "They're baby teeth and they're going to fall out anyway, so who cares!" Primary (baby) tooth development is important for a number of reasons. Normal development of baby teeth is needed to chew properly, which ensures appropriate nutrition. Normal development of teeth is also required for appropriate speech development as well. But baby teeth also are needed to prime the mouth and jaw for the permanent teeth to align well and develop normally.
Generally I encourage all parents to clean their children's teeth at least three times per day. By age 2, you'll notice the molars erupting or "coming out". When you first see that tooth show up (usually on the bottom gum), you can start using a soft toothbrush twice a day. Toothpaste is not recommended for all infants at this point, but if you use some, use only a "pea-size" amount of toothpaste. There are great "baby" toothbrushes available with large handles and small soft brush heads. Be gentle, but make sure you brush all around the tooth. You will need to continue to brush your child's teeth until he/she is old enough to hold the brush safely (even then you will need to supervise).
The complete set of primary teeth is in the mouth from the age of 2 ½ to 3 years of age... Make sure you brush well in the back. During your child's dental visit, we can review technique and check any areas you are missing when brushing.
Dr. Mismas says, "It doesn't matter how many times you brush but how WELL you do it."