My dentist told me that early tooth brushing should be more about establishing a good habit than about getting the child's teeth clean. We tried several children's tooth pastes before settling on one that both my children (twins) liked. It was Tom's orange flavored, from the health food store. Before we found a tooth paste that they liked, we brushed with water. Even if you only get the front teeth on the outside to start, it's the making it into a good habit that is important.
To make tooth brushing fun, I used to brush animals out of their mouths. I would ask them to open wide so I could be sure there were no elephants hiding in their mouths, and then spot one and brush it out with the tooth brush. I varied it each time, so I might be looking for elephants and not find any, but spot a tiger. I made it very dramatic, and there were many giggles. Eventually they started brushing animals out of my mouth at tooth brushing time, and we all looked forward to brushing our teeth together.
As they got older and could brush their own teeth, we did "mirror games" brushing teeth. They would face me and mimic the brushing movements I did, and I'd make different silly noises to go with brushing different areas.
I think their dentist, whom we visted twice a year, was also instrumental in getting them to brush regularly, by praising them and telling them to keep up the good work when they were younger, and pointing out areas that they needed to brush a little better when they got older. They liked to please him.
Good luck!
L. O.