My brother is a dentist and he would tell you that no child under the age of seven can brush their teeth 'well' by themselves. There should always be mommy's turn, then your turn. But he doesn't have kids yet
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At night, I brush DS's teeth in the bath with my arm wrapped around the back of his head, since he tends to just keep tilting it back. We brush for a count of ten for top and bottom, but I think we started with a count of three. I usually count pretty slowly, unless he is in a mood
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In the morning (we just started this recently) I brush his teeth while holding him up in front of the bathroom mirror. Then I let him have his turn while I brush my teeth and I kneel down so he can see me. He thinks this is hilarious and just giggles the whole time.