Fill up with fiber. Few kids crave a fiber-rich meal. But fiber is filling and when combined with drinking plenty of water, helps prevent constipation. A high-fiber food has 5 grams or more of fiber per serving and a "good" source of fiber is one that provides 2.5 to 4.9 grams per serving. Some fiber-friendly foods include cooked navy beans (9.6 grams of fiber for 1/2 cup), a medium baked sweet potato with skin (3.8 grams) and bran flakes (5.3 grams).