I'm not sure why the two fight, but my hens sometimes fight when they are trying to protect their chicks.
Is there a reason why you want to separate the chicks from their mother? If they are fully feathered and perching on their own this is definitely an option, but they can also be introduced to the flock at this age as well.
If you do separate the chicks, you could try it at night, so the mother isn't as stressed. Make sure they are out of hearing and sight range, or else they will constantly try to get to her. If she is ignoring them most of the time then she probably won't be too stressed by the change and the chicks have each other. My mother hens have abandoned their chicks before they were a month of age and the chicks do fine.
I hope this helps. Best of luck!