Give them small servings of everything with seconds of what they like.
Don't serve a lot of one veg.
Make it fun- think duck pancake rolls with shredded veg and dipping sauce.
Mine ate things like tacos and wraps that you stuff yourself. Little bowls of sweetcorn, cherry toms, cucumber sticks, cheese, grated carrot, celery shreds, shredded lettuce- build your own.
Anything served with a cocktail stick to stab and eat- marshmallows, pineapple, olives, strawberries, cheese etc. let them make mini kebabs. Have cheese sauce or melted chocolate so they stab and dip!
Celery boats- fill the middle of the canoe with something they like- peanut butter, cream cheese, cheese spread.
Think in terms of pairing what they like with what they don't, and make it fun.
Home med Veg soup- pea and ham, tomato, French onion etc. serve with a little soup and some cheese toast cut up and floating on top as croutons.
Get them used to a bit of something healthy with their treat foods, and gradually increase the ratio of fruit/veg to treat.
Make their own slush- frozen fruit and juice or milk blended up.
Top their own pizza from a selection of toppings.
They like swing stuff