I would make sure your broody hen and future chick be in an area inaccessible to the other chicken and the cockerel. I like to put mine in crates so they're not disturbed, letting them out once a day for food/water/dust bath/pooping.
**edited to add: when I say in a crate, I mean until the momma hen is ready to get up and leave the nest with her baby. I then have a separate secured area where the hen and chicks can see the flock but nobody can touch them.
I keep mine separated from the flock until the chicks are around 10 days old- because by then they're much more aware of their surroundings and have long enough get-away-sticks and instinct to get out of the way for the possible tussle with the other hen/older chicks/cockerel in establishing herself and her chick. Also - if you can get a 2nd chick so it will have a sibling- things will go much better when the momma hen weans the chick(s).