Let it go for now. If you force her to let you brush her teeth and do it while she is actively physicall protesting you both will be involved in a power struggle. The kid always wins.
My grandkids pediatric dentist said that what is important at this age is for them to develop the habit of brushing but that doing a good job isn't expected until they're older. Your daughter is now proud to be brusing with Mommy and Daddy and this is very good.
Later, say when she's 21/2 to 3 introduce your help in a casual positive way. Perhaps make it a game. Go slowly until she mostly agrees to your doing it. My daughter, when her kids were that age talked about what she was doing in a cheerful voice. Sometimes it worked and other times the kids would have none of it. When that happened she just let it go.
My grandson loved and still does love brushing his teeth. Unfortunately that created a new difficulty. He really just wants to eat the toothpaste. (:
Another idea is to name some of her teeth and have conversations with them. You could say, I want to see Mary tonight. What do you suppose Mary will tell me? Then use the toothbrush to talk with Mary. You be Mary's voice as you brush away. Kids love this sort of silliness.
My granddaughter and her mother have been having difficulty getting homework done. When I took over My granddaughter, at 7, didn't want to talk about it. She was holding one of her Littlest Pet Shop toys and began having him talk for her. I talked with the little animal who gave her the info and she relaxed. I'm going to use that playful way with other issues.
My granddaughter did have a couple of caries in her baby teeth which the dentist did fill. But he said that her teeth are in good shape and not to be concerned about the brushing until her permanent teeth came in.
I've seen babies with a mouthful of cavities. For most of them they slept with a bottle in their mouth. For many of them it was a bottle of Kool Aid full of sugar.
One easier way to keep her teeth clean is to have her either drink some water or to at least rinse her mouth out with water after meals but especially before going to bed.
I just remembered something else my daughter did. She used the prestrung dental floss and flossed their teeth. They couldn't do it for themselves which helped them to allow their mother to do it. The floss holder had animals on the handle.
If you really want to brush her teeth better than she can you might try using a different tool than a toothbrush, perhaps q-tip or a disposable brush whose head is foam. Tell her that this is the next step now that she can brush her teeth so well. But it is a step for mommies to do. I need your help to sit still and hold your head this way. And then praise her for the help she has given while you did this. Perhaps the new "brush" could be "The Inspector."