While I agree that feeding the ducks can be a good way of introducing young children to wildlife, I don't think it should be done with bread. (for all the reasons stated in Jason's post).
I'm all for places that provide bags of seed/grain etc. I think bread is used because it's easy for little hands to hold.....but parents/grandparents really need to change their ways and stop encouraging it!
At weekends/school holidays during the Spring/Summer, the river that flows through Bakewell is often full of bread, pastry, cake etc. I've even seen doughnuts floating along! After a few beak fulls, the ducks just lose interest anyway!
It annoys the life out of me and apart from the fact it looks disgusting, I dread to think what harm it does to the river over a period of time.