Well, we all have our different opinions and methods.
I personally would separate the hen, and feed her and the hen chicks crumb as suggested by others. Why change everyone's feed?
Well, it OP's decision.... I am advising this for the best for the chicks.
I have been told that I am wrong...
Well @DobieLover, my hens tried to hurt the chicks of a broody- its just not worth the risk.
And high all flock is OK, layers is better for layers (I am NOT asking to be argued with please) in my opinion, and switching foods for the broody would be best.
I think it is @chuckachucka's advice...
@DobieLover, you do know chicks have to eat chick crumb and the other hen needs grown feed, right? And the hen could well hurt the chicks.
Using chicken wire like @chuckachucka said is the best option.
You need to give the chicks chick crumb and the broody can eat that too. You will need to separate them because of feeding separate foo.
Enjoy, it will be exciting!