T.D.
First if you have well water you need to have it tested for flouride content. You can do this through your county. If your water has naturally occuring flouride then you don't have to worry about it, otherwise you need a flouride supplement. It is by prescription and comes in a chewable w/ vitamins so if she takes a chewable vitamin she can take this. If you need, it also comes in a liquid but it takes kind of yucky.
As for her brushing on her own, you can let her start but you do need to finish for her. Try using one of those kids "spin brushes". I know that it's made it easier to do my almost 2yr olds teeth. I have friends that have done the same with much sucess.
Hope this helps!