The omega-3's in fish, especially DHA (docosahexanoic acid) are thought to be responsible for the cognitive health benefits. The fish with highest DHA concentrations include mackerel, salmon, sardine and herring. Of these, probably salmon might be the best source for children, the others are pretty fishy. If concerned about omega-3 levels, many turn to children's omega-3 dietary supplements, but again, here, look specifically for the DHA content. total omega-3 numbers can be misleading - it's the DHA you're looking for. And there are vegetarian sources of omega-3 (DHA) too. These come from algae.
RE: Some Fish That Can Improve Children's Brain Intelligence