My birds took awhile to learn to eat their fruit and veggies and are still stubborn about it at times
This is certainly a common struggle and there are many threads dedicated to it. I'm sure you'll find many helpful responses on those as well as this one if you can't find what you're looking for.
Here's one with lots of creative ideas on mixing up the diet with pictures, which is always helpful https://talkbudgies.com/diet-nutritio...t-2-going.html
Here are a few things that I've learned that make it easier
1) I keep the veggies in a food dish so they have the freedom to take a few bites whenever. Just make sure you don't let the food spoil.
2) Chop up the fruits and veggies. My birds won't eat anything that's from the whole fruit or veggie. They will sort of nibble on it, but they won't actually bite and swallow it. I've found a cheese grater is a great way to quickly separate the food into bite-sized pieces for them.
3) When I first started, I mixed in some millet or some of their "treat seed" in with the chopped up bits. They became much more eager to eat their veggies and I was able to slowly wean them off the seed mixed in. Now they get excited about the veggies themselves!
4) Your budgies may also be picky eaters like mine. I always have the best luck with carrots (and they're the easiest to put through the grater since they're hard). They refuse to touch bananas, tomatoes, or anything that's really squishy.
Good luck! Please keep us updated