When I asked Mr. Stinky to brush his teeth he said, "You can't make me!" and I said, "You are right." and continued with what I was doing.
We cannot physically force our kids to do anything, BUT we can implement logical consequences. The logical consequence in our home for not brushing your teeth is no sweets.
Mr. Stinky is learning that when I am so agreeable he is not going to like the consequence. He gave me a funny look and scurried into the bathroom to brush.
Have I mentioned that I LOVE Parenting with Love and Logic? It sure makes my life easier.
When your kids ask for a treat put on your most empathetic/sad voice and say, "Oh, that's a bummer. (Put arm around them.) Boys/Girls who don't brush their teeth don't get anything sweet." and then walk away.
Whatever you do DO NOT threaten this consequence at the time of the brushing confrontation.