She isn't old enough to take interest, probably! You can try using a washcloth rather than a brush. But a child who can't use a spoon or fork won't be able to "get" a toothbrush.
Don't worry about it. Keep her diet processed-sugar and -grain free. Cavities and decay are an "inside" job.
Dr. Judd has some great points about tooth health and preventative dental measures we should take (hint: no fluoride, no glycerine-containing toothpastes).
Mine started liking their toothbrushes around 18 months!