Yes, they can eat other things.
If they were out with mom, they'd be munching on grass and bugs.
They would also be consuming whatever small rocks they need to grind up this food.
If your chicks are not outside in the dirt, then you'll have to supply some grit for them before you give them food other than starter.
I started giving mine tomatoes and squash at that age--they went nuts over the tomatoes. I did buy baby grit for them as well and they do munch on that quite often. They also love those baby forage cakes, i found them at tractor supply--