First of all, don't use whole flaxseeds. They'll pass through the body undigested and won't do any good at all. Buy fresh flaxseeds (preferably organic) at a health food store; they are inexpensive. Keep them in the refrigerator. Before use, grind the seeds (about a quarter-cup at a time) in a blender or coffee grinder that you reserve exclusively for this purpose. Store the ground seeds in the refrigerator in an airtight container, and use them up within a few days. (If they start to smell like oil paint, they've gone bad and should be thrown out.) Sprinkle a tablespoon or two of the ground seeds on cereal or add them to salads, or mix them with your child's peanut butter. A teaspoon of ground flaxseed should be enough for a toddler.
Ground flaxseed provides alpha-linolenic acid (ALA), which the body converts to the same heart-protective omega-3 fatty acids found in salmon, sardines and other oily fish. They also provide fiber and are one of the richest dietary sources of lignans, a class of phytoestrogens thought to help protect against breast, prostate and colon cancers. However, flaxseed isn't as good a source of omega-3s as fish oil, which contains the pre-formed omega-3 s that the body needs. You should be able to find a pleasant tasting fish oil formulated especially for children. You can start giving children supplements after they begin eating solid food.
Andrew Weil, M.D.