We use the Nordic Naturals line, too; we use the ProEFA fish oil supplement. My 3YO has been getting it for almost 2 years mixed in with his morning smoothie. My mother used to give us cod liver oil - nasty stuff!
I haven't taken the ProEFA since getting pregnant, though, cuz my prenatal vitamin comes with DHA - it's the StuartNatal Duet DHA.
The way I see it is that it can't hurt to get more DHA, especially since I don't eat fish too often. So if it helps my body, great, and if it helps DS thru breastmilk, bonus!