How to prevent one mother hen from attacking chicks of another hen?

I have two hens. Both have chicks. The one hen has 7 chicks that are one week old. The other hen has two chicks that are only two days old. The hen that has the 7 attacked the two day old chicks severely pecking the head of one chick, almost to death. When I saw this I immediately seperated them (hen & two little chicks) into a nice roomy cage for the night. All my chickens are free range and I have about 20 acres for them to roam around on, barn, house, woods, pastures & garden. They generally stay around the barn, gardens & house. How can I prevent this from happening again? I don't want to stand guard 24/7. This morning I placed the one hen with 2 chicks into a stall, closed the doors but kept the windows open. That way she and the two chicks could scratch & feed til their hearts content without anyone bothering them. The one chick is still alive & walking but almost scalped. But it is eating, cheeping etc. Other than looking bad, i think it may make it. Any suggestions on keeping the peace between everyone?
TIA
Jan