I tried flaxseed oil with my son but he had an allergic reaction to it--broke out in hives--not life-threatening but an allergic reaction nontheless. If you are wanting to give flaxseed because of the omega-3 oils in it--you can also give fish oil that has been purified and distilled (to remove impurities such as mercury found in fish). This (below) is the type that I have used and used with my son--I add it to his hot cereal in the morning--it has as slight lemon flavor but he eats it willingly.
If you do go with the flaxseed--I recommend using the actual flaxseed and grinding it into powder (this can be done with either a coffee grinder or a blender very easily). As someone already said, the powder can then be added to all sorts of foods very easily (it can also be used in baked goods). It has a nutty flavor but isn't overpowering. It is very important to use it as a powder as the seeds themselves aren't easily digested and will be nutritionally worthless unless they are ground up.
A brand of flaxseed that is readily available in HK (in most Welcome, Park n' Shop and other local grocery strores) is Green Dot Dot--all of their products are organic and are reasonably priced. I also think that they have their own shop in the Central MTR station.
The oil is a bit more overpowering but can be blended with different semi-liquid foods. I doubt most babies or children would willingly drink it on it's own.