Here is a list of suitable treats, just make sure to provide chick grit in a separate dish so they can digest the fibrous material. I give treats once they are over a week old.
I think they should be able to eat whatever you give them now. Make sure you have chick grit for them. I wondered the same thing then thought a wild duck hatches its gotta eat whatever mamas eating so it cant hurt em to feed em fruits and veggies at any age. Mine are about 2 months old they dont like whole corn until it's been wet and soggy about a week or so but I let them out of their pin to run the yard every day. I tried different veggies and fruits to they didnt really care for any of it.