That called for a new routine.
Sometime every morning I put the small kids in the pram/stroller for a nap. I now do that and give them a toothbrush each. They immediately starts to suck or chew on them. They each seem to enjoy to see that the siblings have the same thing in their hands and mouth as they have themselves. I then borrow each brush for a few moments and brush the teeth, and no one seems to protest very much, possible since they can see that everyone get the same treatment? :)