It can be toxic, especially commercial cod liver oil because its heated and artificially cleaned. With fermented cod liver oil maybe bit diffferent, its more safe but taking excess of cod liver oil may lead to overdose for vitamin D and children are way more sensitive than adults. same for Vitamin A, it also high amounts of A in cod liver oil and overdose for A can be serious for children.
Dont mean to scare you thats why I think its smart to consult doctor to see what doses are good for certain age.
I personally take Omega 3 but I try to stay away of Cod liver oil just because nature of it, i.e. extracted from fish liver, Im not sure about all the toxins getting with oil. I do take at least 3g of high strenght Fish oil daily so I do support of taking it daily as its necessary, just not a fan of cod liver oil.