My main concern would be with the contaminants in the fish. There is lots of into out there about which types of fish have the least contaminants, and whether you should get farmed or wild, etc. There is also info on location -- e.g., salmon caught in one part of the world are more contaminated than those caught in another part. If the sushi place can tell you where they got their fish, and if it is farmed or wild, you can make a good decision about probable level of contamination. I'm fairly certain that things like mercury, heavy metals, etc., affect children at much smaller levels than adults. I mean, obviously we don't want it at all, but I am more concerned about exposing young children to it than I am about exposing myself to it.
That said, we eat sushi occasionally, but I haven't given any to dd yet. Again, due to the contaminant concern (I have not bothered to ask where they get their fish because we eat it so rarely).