It is the EPA and DHA that are beneficial in ADHD. Because fish oil is high in vitamin, which is cumulative and can be toxic in higher doses it would be best to use the extracted EPA/DHA rather than fish oil itself. Especially in children.
If you do go with fish oil stick to cod liver oil. I don't trust salmon oil. Too many of the salmon are farm raised and these salmon are full of heavy metals from their feed. Cod from deep cold cleaner waters.