As suggested by the child nutrition specialist, more dietary fiber is not good for the children. If children eat whole-wheat bread in two serving is good enough for them to get good amount of dietary fiber.
As per doctor recommendation a three-year child needs the grams of dietary fiber equal to a child's age plus five. It means 3 year child require 6 grams of dietary fiber per day. It will come from any food contains fiber. You can serve either fruits and vegetables or servings of whole grains, along with other refined grain products.
Fiber has many benefits like it helps children to fight against constipation, for the longer run it will also helps to lower the risk of some cancers and heart disease, which tend to develop later in life.