They must learn to eat off the ground in the wild. Instead of placing the food into their mouths, hold the food barely out of reach with the tweezers, drop the food right before they grab it so they see it fall onto the floor of the box or cage. Eventually, they should learn that the food on the floor is the same as in the tweezers. Do this with the dog food as well as mealworms. Seed can be mixed with the dog food to slowly wean them to seed.
Mealworms are important because they teach the fledgling to catch live food.
As the baby develops feathers and starts hopping around the box, it is time for flying lessons. Eventually, after several days, you can let the bird flutter to the floor. Flying lessons allow the fledgling to strengthen its chest muscles as well as learn to land properly. At about 4-6 weeks of age, the fledgling should be fluttering around the box and be undergoing flying lessons. You can provide free choice food and water in jar lids.