Make it fun by asking your kid to show his/her favourite toy how to brush. Then you can playfully do a voiceover of encouragement and make it seem like it is coming from the toy.
To illustrate the point, Cookie Monster can say: "Me want to brush teeth but me no know how. Who will show Cookie?” Then you can start brushing your kids teeth in front of Cookie Monster as a demonstration and then urge your kid to brush the toy’s teeth after you are done.