There's no consensus as to which is the best fish oil to use. But do check the amount of fish oil in it, as the ones that are more like sweeties contain very little fish oil.
I go for one with a very high DHA content. But others recommend a very high EPA content :)
They come in all sorts of types. Pills to swallow, ones with flavourings, and plain flavours.
DS1 will take an 'orange flavoured syrup'. But I find it has a nasty after tase.
DS2 is quite happy to swallow pills.
DD likes tuna. So I give her a snack of tuna and pour a natural flavoured one over it - she doesn't even know she's taking it. (Because the natural flavoured one tastes very strongly of fish)
Someone else told me you can rub the oil on the inside of a childs wrist and it will be absorbed - I don't know if that's right or not.
Popular brands are EyeQ, Nature's Own, Nordic Naturals, Boots.....
You need to try and see what you can get your DC to take. But do make sure it has lots of fish oil in it.
This is the book that converted me.
It has made a very noticeable improvement in stress levels and concentration for DS1 and DD.