Welcome to the forum!
Babies should mostly get live small animals like live blackworms, small crickets, small earthworms, mealworms, etc. That's what they would eat naturally. Many babies won't even eat pellets so it's good that yours do as the pellets are needed to be sure they get all their vitamins. The babies should be introduced to various fruits and veggies from the beginning but may waste most of them, at least at first. Good first things to try include shredded carrot, kale, cut up grapes, parsley, and so on. Yes, those foods will make a bit of mess which is why you have to net out the food after about an hour if they don't eat it. It helps to feed them in another tank or, if using a small tank, to change the water as often as daily. If you are doing daily water changes, you can leave the food in there. I did the daily changes for just the first few months when my various turtles were in a plastic storage tub with just a few inches of water.
for more on housing and feeding.