T.
M.,
At first I would lay my kids on their back between my legs (me sitting on the floor and their head between my thighs, their feet between mine), pin their arms under my legs and brush their teeth. If they'd clamp their mouth shut, pinch their nose for a few seconds and they'll open their mouths.
My older child has autism and is facially defensive and brushing his teeth was a real nightmare for a long time. But I didn't want his teeth to rot (he's 5 now and still cavity free!) so no matter how unpleasant, we brush teeth. My 2 y.o. was only resistent to brushing his teeth for about 2 weeks then he decided he didn't like being held down and now he just opens his mouth. LOL I also found that the kid electric tooth brushes with characters on them (Dora, Spongebob) helped a bunch. Both of my kids are more willing to brush their teeth with the electric toothbrushes.
T.