Im sure you can get the little finger things that you put on your finger and brush their teeth with rather thank sticking the brush in their mouth.
I know it sounds terrible but i used to get really grumpy with Ashlea and she'd end up in tears, but would open her mouth to cry and i could brush her teeth then. I was worried about the negative association with the tooth brush though, but was paranoid about cavities cause I knew a little girl who had her front tooth turn black and fall out when she was 4. So in the end we would put on her favourite song at the time (Robbie Williams, Something Beautiful) and that became the 'tooth brushing song'. We'd put it on and get all excited and say 'quick quick teeth brushing time' She would run to the bathroon and happily let me do it then.
Another idea is to tell them if they let you brush their teeth then they can brush yours, or brush your teeth together. Ash also had a doll that always had a bath with her and her mouth was open so i bought a little toothbrush and if she let me brush her teeth, she could brush her babies teeth.