that depends on their diet. most kids need supplemental vitamin d because it not found naturally in many foods and the sun is not reliable. the american academy of pediatrics recommends supplementing with d for children not receiving enough through diet. it would take 6 cups of milk to get the 600iu recommended -- and that would be too much milk for kids anyway. as for omega-3 fatty acids i assume your referring to dha which is important for health and brain development in babies and young toddlers. if a toddler eats fish about twice a week (small portions) than it's not needed. if he doesn't eat any fish you may want to consider supplementing. fortified foods have some but not as much as in fish.
should toddlers take vitamins with omega 3 and vitamin d3