Paul, there is a scientific basis for the idea that vitamins/minerals in many supplements aren't as readily absorbed. For example, it's true that the magnesium in a supplement is identical to the magnesium in food, but the vitamins in food are often bound in larger molecules that can cross membranes that the isolated compound cannot.Vitamins and Minerals for Healthy Children - WebMD
Megavitamins -- large doses of vitamins -- aren't a good idea for children. The fat-soluble vitamins (vitamins A, D, E, and K) can be toxic if kids get too much of them. Ditto with iron.Is There Really Any Benefit to Multivitamins?
Is There Really Any Benefit to Multivitamins? A recent look at multivitamins by Johns Hopkins researchers shows that there’s no proof of benefit, but there is evidence of possible harm from high doses of certain vitamin supplements.Vitamin Supplements and Children - HealthyChildren.org
Following are some of the vitamins and minerals necessary for normally growing children, and some of the foods that contain them. Vitamin A promotes normal growth, healthy skin, and tissue repair, and aids in night and color vision.DHA Benefits and Side Effects for Children - Verywell Mind
DHA supplements are often available in flavored "gummy" form to make the supplements palatable for children. Bottom Line Due to the lack of research, it's too soon to recommend DHA as a treatment for any condition in children.