My dd is the same age as your Sonja (beautiful name, btw). What we do might give you some ideas. We stand at the sink facing the bathroom mirror with me standing behind dd. I brush her teeth and describe what I'm doing while she watches in the mirror and listens. Then it's her turn. She gets practice and direction; I know that at least during my portion that her teeth were properly brushed.
Maybe? Maybe not?
Maybe? Maybe not?