Okay, so I'm copying this from Vegan for Life by Jack Norris, RD and Virginia Messina, MPH, RD. I hope that satisfies any copyright infringement nonsense.
They say: "There are no studies measuring levels of the long-chain omega-3 fats EPA or DHA in vegetarian or vegan children. however, we do know that many healthy children have been raised on vegan diets without supplements of either the long-chain omega-3 fats or the essential fatty acid alpha-linolenic acid (ALA). It's possible that children who are vegan from birth are more efficient at manufacturing DHA and EPA. But until we know more, we recommend a DHA supplement of 100 to 200 milligrams per day for children." (page 159, paperback edition)
Freya is still breastfeeding to a large degree, so I'm not too fussed yet (both the Vegan Society and Norris/Messina don't see supplements as necessary for nursing/formula fed infants). However over the next 4-5 months, it's likely that we're going to nurse considerably less as she eats more big people food, is busy in that toddler way and god help us, starts sleeping better and nursing less at night. So, this is something we have to consider shortly. Right now, she does a D supplement and soon (for the same reasons outlined above), we're going to start B12.
FWIW, Norris and Messina recommend an Iodine supplement for children/teens as well.