Have you talked to your dentist about this? Our family dentist said to let my daughter brush her own teeth. Trying to force it could result in injuring the soft parts of a child's mouth. Even if they just chew on the toothbrush, that's enough. Again, this is what our dentist has said, and I would suggest talking to your dentist before you try to hold the child down or pry their mouth open.
two year old won't cooperate with teeth brushing, I am stumped. - Page 2
I don't know if you ever read Dr Ellie's blog, but I like what she has to say about children's teeth. It's about good oral systemic health and creating that environment...neutralising acidity with xylitol. I barely brush my 2 year olds teeth, but give him xylitol gum to chew on (which he loves and never swallows accidentally) and then give him xylitol in his water.
You can read more on her blog. http://askdrellie.blogspot.com/