A.C.
My 21 month old daughter started doing this a couple weeks ago...what I found works best for me is first of all, pretending to brush my teeth with her toothbrush (or you could even show her how you brush your teeth with your own toothbrush so she sees that mommy brushes too), making fun noises to go along with it. If that doesn't work, I will start singing a song, like "this is the way we brush our teeth, brush our teeth, brush our teeth, this is the way we brush our teeth, so early in the morning (evening, afternoon, whenever it is)" and keep repeating it and then she'll usually open her mouth for me because I make it fun. Hope that helps!