Are fish oils safe for toddlers?

I have a 3 1/2 year old son with ADHD symptoms and a 16 month old son with a delay in language development. Both were adopted from orphanages in Russia where nutrition was very poor. I've been reading a lot about EFA (essential fatty acids) and how an increase could improve ADHD behaviors and could possibly contribute to improving speech. I've read several articles on this. My concern would be mercury. Can anyone recommend a supplement with EFA or DHA that would be safe for them to take? Flax seed oil, possibly?
Any help would be appreciated.

Thanks,

Lynne