Ok, in the mornings I literally will not let them get dressed and I won't let them out of the house until they have let me brush them properly. Awful, boring, tedious, but effective. I often pretend to phone somepne to say "Sorry, no, we can't meet you at the park today as Olivia and Charlie will not brush their teeth properly"
( I have carried out the threat only twice.......)

Night time is heaps easier.......in the bath.......DH makes them brush their teeth first and coaches them through it. Only when he considers that they have done a "proper job" of it are they allowed any bath toys......he is strict on this, and it only took one night for Olivia to "get it" and she now brushes properly every night........Charlie tries his best too.......

Not sure if that helps any?