I just responded to something similiar this morning so i am going to copy and paste what i wrote:
"Well, I had to get creative and make teeth brushing fun for my daughter, we sing and dance while we brush. First I brush for her while I talk about the foods we've eaten that day, to get to certain spots I will say "OH! OPEN WIDE, I gotta get that banana off your back tooth, oh I see some chicken" ect and she thinks it funny that I am brushing the food away. For round two when she gets to brush, we sing. I take somgs we know and just put new lyrics on them, For example if you've ever heard the song "I like to eat, eat , eat, apples and bananas" I sing "I like to brush, brush, brush, brush all my teethers!" There's a ton of songs I sing, Just make it a fun event, sure brushing her teeth takes a little longer, but it's done good. I also use the Toddler training toothpaste, flouride free.. One negative to that is I can't seem to get her to want to spit it out!"
If she closes her mouth tight stop singing and having fun, only provide fun when she is cooperating.
I personally wouldn't feel comfortable holding my daughter down, but everyone has their own ways. I feel like this would make brushing teeth such a negative experience that it wouldn't be instilling the habit in her, which is what the first goal really is. Also, holding her down could really stunt her trust for you. Instead, until you feel she is really getting her teeth cleaned, it may be a long process, cut back on sugars and sweets and anything that will affect her teeth, especially close to bedtime.
Good Luck!