Fishcakes always went down well.
I make my own - cod, spring onion, pancetta and cheddar/salmon, spinach and garlic & herb cream cheeses.
They all wolf them down.
Fish goujons/fishfingers were a good fallback. Easy enough to make and freeze.
Tuna pasta bake.
Haddock and spinach risotto (BBCgoodfood)
Cod/salmon with a pesto crust.
Cod and chorizo stew.
I do find the BBC goodfood website to be a good source of inspiration. Or kids cookbooks - the recipes are simple and the kids like it!