I currently have 3 "chicken jails" out in the yard. Two have hens with their chicks, and the third has my 4 orphaned chicks. The momma hen died (found her dead) when the chicks where 2 weeks old.
Each "jail" is completely screened in with a chicken wire "skirt" so no critter can dig under. They are on grass--but by now, the two sets of chicks are 30 days old, and the grass is scratched down to dirt. Each has a wooden nest box, waterer, and partially covered so there is always shade. I give treats on paper plates and have all the other food tossed in. The jails are 8' x 8', 8'x6' and 8' x 4'. The orphans are in the 8x4. I wonder if it is too small and somehow stressed out the momma hen. Have no idea what killed her. I have another hen sitting on eggs in a nest box, and those kids are due in two weeks. Guess I will construct another jail if the time comes.
My 8x8 jail was originally a big dog pen that was built with 2x8 pig fencing and I wrapped it in chicken wire. Here is a pic of it from last year.
https://www.backyardchickens.com/forum/viewtopic.php?id=68782
The 8x4 is an extension to one side of the 8x8. The 8x6 I built using 2x4's and followed the chicken tractor on this link.
https://www.backyardchickens.com/web/viewblog.php?id=1642-tractor
My birds are free range, but in the last 2 years (only owned the property 2.5 years) I lost so many mommas/kids to raccoons and roadrunners and hawks, that decided I'd "jail" the little families until the kids were nearly full grown. I have several nest boxes secured at 4 to 5 feet off the ground so the hens can sit safely. When the chicks hatch out, I move the family+nest box into a jail.
In the jails, when the kids are around 2 weeks old, I place the nest box up on crates and add ramps. Then at 4 weeks, I add some tree limbs in the jails and find the chicks are always busy flying up/climbing, etc. Around 8 weeks, I'll let them out supervised. Soon then, I'll open the tops so if they can fly/jump/climb up the limbs, they'll hopefully follow all the big chickens and and go up in the trees. That's the dicey part of the whole thing. At least they'll be 2+ months old and be physically able to get themselves in the trees. By then the mommas will be cutting the kids loose too. The jails are right by the roosting tree so they can see everyone heading up the tree to bed at night.