In Chicken World there are pecks and there are pecks. Chickens vocalize among themselves, but a lot of communicating is done with pecks.
You need to watch your broody and determine how hard she's pecking the chicks. Watch her beak. A light "scooping" peck toward herself means "get under me where it's warm and safe". A swift but light peck straight down on the back of a chick means "get with the program and quit being naughty, or "listen to me, I'm your mother!"
Later on as the chicks begin exploring, she may discipline them with a hard peck to the back. They're older now and can take a harder peck, and besides, they probably have earned it by stepping out of line. The other chickens will also discipline chicks in this manner.
A broody that is psycho and wants to kill her chicks is going to be screeching while she hammers away at the poor little things. There won't be any doubt that she means them harm. Should you ever see that, remove the chicks from her immediately.