I don't think it would have anything to do with potty training. But one thing you could do is give her choices. First ask her if she wants to use this toothpaste or this toothpaste. Wait for her to answer (about 10 seconds), if she doesn't select a choice, then pick for her. Then ask her if she wants to use this toothbrush or that toothbrush. (yes, get her a couple of each) Then wait for her answer (same thing as before). Let her feel like she is in control a bit, but give her choices that you are willing to work with. Ask her if she wants to stand on the step stool or sit on the counter...wait for her to answer. Then ask, would you like to brush your own teeth or me brush your teeth. Hopefully she'll allow you or herself to do this.
Now, let's say that she refuses to brush her teeth after all the above questions. Let it go and then (this may seem harsh, but effective) later on perhaps your husband could go by Dairy Queen and get an ice cream for you and him. She'll want some, but you aren't going to let her have any. You say, very empathetically, 'OH, this is so sad. I let kids who brush their teeth have treats." And leave it at that. If she asks again, be a broken record. Don't get angry. If she cries and throws a tantrum, which any normal child would do in this situation, say, "Uh oh, sounds like you need a little time in your room" and then escort her there. You can then ask her if she wants the door open or closed. Do you see where I'm going here?
The treat should make her want to do it. But if her memory is short lived...try it again with a different treat...candy bar...what ever it is that you know she would like and repeat...I give treats to kids who brush their teeth. No lecturing or explaining. Actions speak louder than words.
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