I used to make cooking a meal together a big treat - the youngest chopped mushrooms with a plastic knife, the next one up had a normal table knife to slice peppers, the eldest had a kitchen knife and chopped onions, and one grated carrots. They were all willing to eat food they had prepared themselves, so a stew or lasagne or stew was both a fun thing to do together and a good way to introduce new foods.
If all else failed, I would refuse to give a child something and say ‘you won’t like that - it’s a grown up taste’. That was guaranteed to make them say ‘but Mum, I’m a big boy now. I like broccoli/ sprouts/ olives/ whatever’.