As long as you give them some grit, anything an adult can eat, a chick can eat. Just be careful not to over do it with treats. They are growing babies that need all the nutrients in feed. You don't want them filling up on desserts.
First of all, your Jersey Giant looks like a pullet. Second, it only looks to be about 6 weeks old, not 8. And that crate is too small for three, that age. They are ready for an outdoor coop. This could be the reason for the bullying. Too cramped. They need more space.