I make a mix of chicken scratch, cracked corn, wildbird finch mix, sunflower chip hearts, cockatiel molting and conditioner seed and cockatiel fruit and nut mix, mix all this together and feed a handful a day on the side from their gamebird food. I also provide millet sprays and milo sprays which they love.
Edit to add: And yes, fruits and veggies are a GREAT thing for them...mine go gaga for snap peas in the pod, quartered apples or peaches, lettuces, greens, broccoli slaw, and even hand picked clover when it is in season. (They will bowl you over for fresh clover!)