As far as sushi being an art form, what makes it so is because of the skill of the chef and how he cuts the fish, as sushi and sashimi require the fish to be cut a certain way, angle, from certain area, etc.
For sushi grade fish, there I'd really technically no such thing. I can't speak for japan bc I don't know what kind of regulations they have(and if any country would have them re sushi it would be them, though I also find that doubtful since they seem to rely on actual artistic skill for things like this), but I know in the States and Canada there is no such thing as sushi grade fish. What WOULD constitute as such is because of how the fish is handled after it's caught(exam, it's supposed to be bled a certain way and automatically put on ice), that would determine its quality.
In the states and Canada all sushi fish must be frozen prior to serving as sushi, to kill the Parasites that live in raw fish(some more than others).
I just really doubt that your average Japanese resto here really import their sushi fish from japan; the few average ones I have asked do not. Salmon does come from colder waters yes, so it's imported, but yuck, look how much cold water there is around china. At PNS the salmon always says it comes from so-and-so ocean, not what county.
I would expect that in proper sushi restaurants, as the OP seemed indicate she was looking for, that they would
be using Japanese fish. Then you can afford to be undiscerning for your kids.
Just for me, since we don't go to these places often, I'm pretty choosy what I eat, perhaps moreso for taste and freshness. The salmon is pretty yummy here in most places I've been though. Just more cautious for my kids.