I guess I should add that my ANC pop up is enclosed by electric netting. The netting will still be there when they are big enough not to get through it.
I got an enclosure from Amazon for $68. It has no floor so they are on the grass, but is otherwise completely enclosed with 3 zipper doors. You have to put boards or something on the bottom flaps so they don't sneak out. ANC pop portable chicken run. It is now $64