Farmed salmon is supposedly much higher in toxins than wild salmon. But it is PCBs and Dioxin that, as far as I know, are the major contaminants. PCBs and Dioxin are obviously just as bad as mercury for you. There are claims that other farmed fish can be contaminated because of the water that is used.
Fish that are oily, like tuna, swordfish, bluefish, generally have a higher mercury level than other fish.
So maybe the best thing to do is identify the WILD species of fish that generally contain the lowest level of contaminants and serve them instead. Usually they are the milder white fishes that kids are more likely to eat anyway.