Just inundate them with everything you can that isn't too spicy. Once you pull out the Uni a few times, they will think they will have died and gone to heaven when you roll out a piece of baked haddock with scallop potatoes and steamed asparagus.
I believe by requiring them to try new foods, you'll be planting seeds in their psychie that will flourish in later years. If you can think back, its probably whats motivated many on this forum to make their way into a professional kitchen (sure aint common sense HA). Those early childhood tastes will haunt them in their later years. Hit them with everything, think about more like breaking a horse than teaching a kid something. They will hate you now but love you later. Remember, its takes more than just love to raise a child. Like starvation and unilateralism.