How much OMEGA 3 & 6 can I give my child?

Omega 6 is already in our diets without any need to supplement. Also excess Omega 6 prevents the body from absorbing the Omega 3. So it's not helpful to supplement with 6 at all. There was a really good BBC radio programme about these issues which you can listen to here

I don't know if there are specificities for children, but some scientists like David Servan Schreiber alert about the western diet which is strongly unbalanced with respect to omega 3 & 6. According to him, the correct balance should be 1 / 1 but we eat about 20 times more omega 6 than omega 3. The main risk is to favour development of cancer. His book Anticancer is worth a read.

The stress is to replace Omega 6 intake with Omega 3 intake, and to ensure that the Omega 3 is not just the plant derived short-chain Omega 3, but that it is EPA and DHA long-chain Omega 3, because the benefits are generally from the long-chain variety which cannot be easily made by the body from the short-chain variety. A recommended daily minimum 550mg of the latter seems to be agreed on by nutritionists but not by the government regulations, yet. Because some products that include DHA Omega 3 also include much larger quantities of either saturated fat or trans fats (e.g. milk and margarine) you still have to weigh whether the source is actually beneficial to your health. This is also reported in the BBC radio program that Meg linked.

Keep in mind that supplements are entirely unregulated by the FDA, at least in the United States, and companies can put whatever they want on the packaging and no one ensures that it's accurate. Basically you can't trust the product labeling. For example, some other supplements (eg, ginko) have been studied and found to contain either much more or less than what is on the product label. And even more scary is that contaminants like arsenic etc are found in these unregulated products. I recommend a healthy varied diet and that you save your money.