How is their weight? If they are healthy weight i would gradually start pandering less to their wishes.
Do not make a fuss and do not make them eat anything. This is a child who is quite old so the habits will be really ingrained, it might take 2-3 years to really notice improvement. Ive done this with my DC from age 4 and tonight (now aged 6) she's eaten roast pork dinner & voluntarily served herself new potatoes (with skin on), roast pork, thin green beans, broccoli & peas. She added gravy on and ate the lot!
Involve DC in food preparation. Make clear they don't have to eat it but they have to help peel carrots, chop fruit, stir the sauce, have a go at kneading the dough. Touching and smelling the food without the pressure of being made to taste/eat it will help them reduce aversion. Encourage pretend (or real) cooking eg pasta buffet where you can mix up your own food (put dishes of pasta and favourite/safe foods like peas, sweetcorn, sausage and encourage self serving & mixing their own "recipe".
Example of changes you could make in small increments:
- add one new/different item to the plate. Dc does not have to touch/eat but needs to accept them on the plate.
- make a small presentational change. Different shape or colour of pasta. Cut the carrots a different way.
- serve two foods touching & gradually move towards mixing
- introduce new fruit flavours via juice blends or adding purees to yoghurt
- make meatballs from chicken mince instead of pork.
stop snacks. They kill appetite for meals.
You have to remember it takes 15-20 exposures to make a change. That means if you offered the same 5 new vegetables, 1 each night every week, it might take 6 months to get those accepted (not liked) and that's if you are really, really consistent. When you say you've tried things and they "don't work" have you tried it consistently, repeatedly, for months? Because it doesn't work doing it four or five times. It takes months. It is really hard, changes have to be very slow & take months - you have to see it as a lifestyle change.
Do you all eat together round the table? They need to see you all enjoying varied foods.