urbandad--
if the mom is getting good nutrition, she is probably passing on some omega-3s/DHA will she, nil she. Good for her, though, for still breastfeeding at 20 months and working(? the 5 days he's at daycare) ... be sure to encourage her, if she doesn't hear encouragement from you as backhanded advice (which women often do).
I was vegetarian for a while; I faithfully took flax supplements for the first two pregnancies + nursings and sporadically for the other two; all the kids are pretty darn smart, but the first two had heads literally off the size charts at birth, so I don't believe the page I just googled claiming that flax seed oil supplementation doesn't work ;)!!
If you are very lucky, your toddler might eat flax pills like candy (one of mine did, two of mine didn't, but one of those would eat them when she understood they were good for her, I'm so out of the habit I haven't tried the 4th, which I should now that I think of it). There are organic and non-organic ones; dunno where you stand on that.
My kids' dad used to eat cod liver oil as a treat when he was a kid ... you never know unless you try ;).
Mothering magazine used to advertise every month a fish oil with all the fish taste taken out ... might have been Nordic Naturals? Anyhow, I don't read magazines much anymore, but you can search alternative/natural lifestyle magazines online (Mothering.com is a great site) for articles, or browse them at the store for ads ;).
My kids love smoked salmon (which they get occasionally from their dad, as I don't have money for it) ... I'd figure the omega3s might be killed by processing ... except the Inuit smoke some of their fish for storage, don't they, and they were the reason we found out about omega3s being so important anyhow, so maybe that would work. If we ignore the carcinogens inherent in smoke. (Life: so complex!) Sardines are another one ... dunno if the canning kills the DHA, but my kids like sardines.