I have recommended the Nordic Natural Omega 3 fish oil and Carlson's Cod Liver Oil to a number of friends who have kids . One friend had two sets of twins and she was desperate for one of them to take the fish oil. When she switched to the Nordic Natural brand she suddenly had all of them taking it. If you buy it from the internet, where the stock is fresh every two weeks, and then you keep it in the refrigerator, it should smell like lemony oil. If it has been sitting on the shelf at a health food store, it might be rancid. My kids take 1/2 to 1 tsp either Omega 3 fish oil or Carlson Cod Liver Oil. The Carlson Cod Liver Oil contains some natural Vitamin A and D. It is great in the winter or if someone is fighting an illness.
Here are some links to the flavor concentrates--the company that makes the bubblegum is nowhere to be found but I found another company that has a bunch of organic flavor concentrates that would appeal to kids:
Fish oil & Cod Liver Oil Sources: