If they simply don't like fish, I would give up on giving it to them for a while. Sometimes they need to have a break from the pressure of trying trying trying to like a food that their taste buds just can't stand, and then having it reintroduced after a year can make a huge difference.
My youngest daughter to hate all kinds of beans, fish, potatoes, blueberries, eggs, and eggplant. I'm talking full-on gagging at the mere odor. I didn't push it with her or any of my daughters, and instead reintroduced it at a later time. If I'm cooking and she (or any of my daughters) comes over while I'm cooking to ask what I'm making because it smells and looks good but was something they've hated in the past, I'll suggest they taste it if they think it looks good. They love to "taste test." 9/10 times they've got a "new" food they love. Unless it's my daughter with Autism. She may taste test but it's rare that we find a new food to add to her menu.
With fish, if you really insist that they at least choke down a few bites for the sake of "finishing what you made for supper" then try to incorporate something that you know they already love into the recipe. If that means slathering it with ketchup, let them knock themselves out. Try making a typical tartar sauce and then adding a few tablespoons of Dijon mustard for a nice tangy touch. Make filets into sandwiches on a hamburger bun, plop on American cheese slices, sauce of choice ala McD's, and then cut them into quarters.