When my kids were little, I would have them sing a song while they brushed their teeth to be sure that they brushed for at least a couple of minutes and were thorough. 🙂
I do not have kids of my own, but I have 3 nephews and 1 niece and I have taken care of them a time or two. When it was time to brush their teeth I would have them hum a song or watch a video about brushing.