You can buy kids toothpaste that come in bubblegum flavour and lots of other yummy flavours....come even have glitter in them
Jack loves to brush his teeth in the bath. He started off just chewing on it (which I figure is better than nothing at all) and now he will use a brushing motion. If you are using a children's toothpaste you don't need to stress too much if they don't know how to rinse and spit properly just yet.
Maybe make up a "tooth brushing" song to make it more fun.