The origin of all omega-3s is the photosynthetic center of plants, the chloroplast.

Chloroplast

Omega-3 fatty acids are synthesized as components of the cell membranes of chloroplasts, and no matter whether your chloroplasts come from here

Microalgae. Diatoms in this image.

or here..

Lemon grass, an example of a land based leafy plant.

adequate omega-3 fatty acids are easy to come by if you take a little care with your sources.

Steamed spinach. Popeye had the right idea.

Omega-6 fatty acids come from grains and nuts, such as these

Wheat

and are concentrated in huge quantities in grain oils, such as corn oil, cottonseed oil, and canola oil.

Corn oil

Now, when one of these

Cow!

eats large quantities of grass, they act as biological concentrators of those fatty acids. The butter from grass fed animals, in particular, is a great source of omega-3 fatty acids. If, however, your meat sources eat large quantities of this

Corn

well, corn is a grain and those animals are going to be stuffed with omega-6 fatty acids. Animals, whether swimming or 4 legged, maintain high concentrations of the oils they eat.

In the absence of grass-fed land animals, sardines and herring are perhaps the cheapest, most available source of high quality omega-3 fatty acids, and cans of them can be purchased for perhaps 90 cents at the local supermarket.

A school of sardines.

Mackerel also works, as does trout or wild salmon.

Since omega-3 deficiencies in small children have been linked to learning issues, commercial manufacturers have moved into the gap.  These new products, however, have small servings of DHA at rather high prices ( 30 to 50 cents a pill), and tend to be given in pills along with a small scattering of vitamins (in pill form, a dose of vitamins costs about 5 cents each). Another common sleight of hand trick is to add a small amount of flax to a largely grain based cereal. Omega 6 from the cereal grains are going to overwhelm the small advantage gained by a tiny bit of flax seed.

Barleans, a respectable brand of flax seed oil. Whole Foods has a good store brand.

I have yet another suggestion. When possible, don’t feed your children lots of grains and lots of grain oils (or sardines soaked in cottonseed oil), but rather, perhaps get a little flax seed oil and cut up a bit of a really good tomato.

Japanese black trifele tomato, a good heirloom.

High quality Roma tomatoes can be had at the local market.

The flax seed oil will provide a very useful dose of plant based ALA, the oil will act as an excellent carrier for tomato lycopenes, and further, the conversion of this plant based omega-3 to EPA/DHA will be determined by the needs of the eater. I suspect it tastes better than a little pill, and per serving, feeds more than a pill.

If your young one is a plant hating carnivore, a little sardine or tuna mixed with a stretcher (perhaps a olive oil pesto) works.

Note: the vast majority of the images above come from Wikimedia Commons.