I have tried a lot of things but I have found the following 2 strategies work:

1. I tell them its "Brown Sugar Bread"; so I ask them whether they want white bread or brown sugar bread.

2. I run-out of white bread and so the only thing I have in the house is whole wheat bread. I have found this to be the most effective strategy. They want bread, so they eat the whole wheat bread, and they don't complain.

Of course, the problem probably originated because I did not give them enough whole wheats in their first years of eating. A good friend of mine has always and only given her children whole wheat bread and they do not know it any other way.