DHA, they had a statistically significant improvement in their listening comprehension and vocabulary acquisition. In the study, 175 children were assigned to receive either 400mg daily of DHA or a placebo. The best dietary sources of DHA are cold-water fish such as salmon and sardines. DHA is also found in seaweed and algae.
"Assessing the effect of docosahexaenoic Acid on cognitive functions in healthy, preschool children: a randomized, placebo-controlled, double-blind study," Ryan AS, Nelson EB, Clin Pediatr (Phila), 2008; 47(4): 355-62.