Hi Sarah,
Boy do I know the woes of trying to brush a young toddlers teeth! Our little guy grinds his teeth sometimes. I guess having these new hard things in your mouth makes them curious about what they do. This could be the reason for the worn out enamel.
I also find that when he has new teeth coming through that he is adamant about not letting me brush his teeth. When I'm dealing with a potential tantrum, I just get a piece of gauze with a bit of non floride tooth paste, and use my finger to clean his teeth. Watch out when doing this you may bet bitten! but it does seem easier than trying to stick a brush in his mouth.
You can actually find a baby tooth brush that just slips on your finger that is made out of soft rubber(ish) bristles. You can also find ones that are shaped like a bite with a handle that they like to chew on themselves when teething ask around at a good local baby store for these.