the best source is oily fish. But the problem, as most parents know, is that children don't like it, so Omega-3 supplements are becoming a popular option.
Traditional cod liver oil is not particularly rich in EPA and DHA; it also contains the fat-soluble vitamins A and D, that can be toxic in excess, so high doses are not advisable in the long-term.
In a recent survey, it has been proved that About 450-500mg per day combined EPA/DHA is a reasonable target for both adults and children