I do the eeee - aaaah song with DS, though he's nearly 3. I get him to say "EEEEEEEEEEE" (usually we do it to a little tune, I do it too) while I do the fronts, and then get him to sing "AAAAAAH" - again to a tune - while I do the molars. I by no means get them scrubbed to the level of my own teeth, but it's getting him back in the habit, we had got sloppy about it.
Before we got to the song stage I would just let him try doing his own while I did mine, doing my own slowly enough that he could copy if he wanted to, which seemed to accustom him to the idea of the toothbrush and then asked every day if I could have a turn doing his, he said no for ages, until one day when he said yes and then the idea of the song came to me so I got him to do that, now he finds it fun when I do it so he doesn't mind. He usually laughs, but if he moves too much out of position he's not making the right eee/aaa sound any more so he quickly closes/opens his mouth again - it's great!
When he's older then I will explain about brushing hard but my dentist says that when they are little it's much more important to establish the habit than worry about how well the teeth are getting cleaned, though of course you should try to clean them as best you can. Also to limit sticky foods like raisins and dried fruit, and encourage eating things like raw hard vegetables, e.g. carrot, and sticks or cubes of cheese are very good too.