Despite popular belief, bread is not a good foodstuff to give to wild birds and ducks. This is particularly true for white bread which has little nutritional value and is filling.
My son LOVES feeding the ducks, so we take a jar of bird seed and give them that instead. It is much better for them. There is also an added bonus of not having to be organised enough to buy bread before you take a toddler for a walk. We keep a jar filled up at all times in the car boot so we can go on impromptu duck feeding adventures!
You can buy special duck and swan food and other ideas can be found on this site including frozen peas. You can also download a free duck activity guide to have some fun with the children at your local river or canal.