My kits will get a small sliver of salmon or tuna sashimi to share, occasionally, if we order in sushi and have a bit of leftovers (doesn't happen too often in my house--we love sushi and rarely have any left behind!)
I give it to them ASAP--right after we're done eating ours--and if there's any in their bowl about a half hour later, it gets pitched. The two of them share one piece of sashimi which I chop up into very tiny pieces--and they love it, particularly salmon, which is also mommy's favorite!
A little bit every once in awhile won't hurt them so long as its just a tiny bit and you make sure not to leave it in their dish overnight to invite bacteria. Also, make sure you serve it to them in a ceramic or metal dish that can be run through the dishwasher--plastic bowls can hang onto bacteria even after theyre washed.