When do you start brushing their teeth?

[Ernestina]

And do you use toothpaste? Suddenly occurred to me the other day that perhaps I should be brushing ds's teeth? But am sure he will just want to eat the toothbrush...

I started as they first got teeth tiny tiny dot of toothpaste

As soon as have teeth - even if wants to eat toothbrush it's all about getting used to it.

I have never brushed DD's teeth.
Just gave her a toothbrush with a eeenny bit of toothpaste when she started solids and let her get on with it while I did my own teeth. She learned by copying me.
BTW - at that time DD had no teeth, but I wanted her to get into the habit.

I started before her teeth came through by putting Calgel on a toothbrush when she was teething. I handed it over and quickly she got the idea that if she moved it around her mouth it felt nice. As she got teeth I replaced the Calgel with some baby toothpaste and we've never had a problem brushing her teeth.