Do remember though that the fact that most children lie from time to time does not mean that most children grow up into unloveable untrustworthy adults.
What they will hopefully grow up into are adults who understand when a little lying is ethically acceptable ('oh, that sounds great fun, such a shame I can't come, but thank you for inviting me') and when it is not ethically acceptable (e.g. lying to your partner about the man you went to dinner with).
But to make those distinctions, you need to be relaxed enough to show them that not all lies are equally serious. Otherwise, they genuinely will grow up thinking there is not difference between a few polite phrases about a party and the kind of lying that undermines trust. And as they can't very well get out of the first kind of lying....
I would make the toothbrushing thing a bit of a family injoke, giving them the idea that this isn't really a lie, because we all know that you will try anything to get out of cleaning your teeth, but Mummy is smart enough to outwit you. If you can treat it like that, then you will keep it in proportion and they will soon learn it is not worth doing.
On the other hand, come down on them like a ton of bricks when it comes to serious lying, like trying to get a friend into trouble.
If they can get into serious trouble over tooth brushing, then I think the family may need to lighten up a bit.