K.B.
Well its usually said that until a child is about three or four years old, they aren't going to be able to do an adequate job at brushing their own teeth.
Sometimes singing during activities can help a lot - so that's good that you're doing that. I sing a song to my 21 month old to the tune of "row row your boat":
Brush, brush, brush your teeth
morning, noon, and night
toothpaste and a toothbrush
does the trick
keeps our teeth clean and bright
I talk to her about what areas of her mouth I'm cleaning also. "lets get the top right... now the top left... okay now lets get your tongue. Go ahhhh." I figure that way, when she is old enough to do it on her own, maybe she'll remember which spots to hit.
I know theres also a mouthwash for older kids that they can use before brushing. It puts a blue tint on the plaque on their teeth, that way they have to brush to get it off and you know they didnt miss any spots.... but it wouldnt work on such a young little one.