If they’re babies, it’s definitely a no on the shells. If they’re laying, you can give them the shells separate. I crush them a little when I give shells. They’ll be happy with plain eggs, no need to add anything to them.
Scrambled eggs are great for grown chickens, shells and all. Chicks however usually aren’t interested in treats and it’s essential that they get what they need out of their chick food. I usually don’t give or even try to give treats until they are ready to forage outside.