We think that PVC pipe would be great material for kids to fort-build with. What you see there cost less than $30 and is infinitely adjustable, and you could use tarps, blankets, sheets, whatever for walls. If the whole thing caves in due to design flaws, it's so light it wouldn't matter! Some zip ties and chicken wire rounded off the supplies we needed.
This is the day we moved the main chicken coop to a better spot in the yard, pulling it with the pickup and rolling it on logs. (Yes, Zoralee is referred to as Zoey, a common mistake since she has a little friend named Zoey.)
all settled in and roosting for the night