Recently we camped with friends and the boys' tent came without poles. I did this out of necessity but it worked like a charm. In fact a small tent with no poles is now part of my hiking gear/ Go Bag (which you can see here), since it weighs very little.
I apologize if this is way too obvious, but when I looked up the subject I saw an Instructable that involved 100 feet of cordage and is an impressive feat of engineering for sure, but it seems like more work than most frustrated campers would want to do.
So here it is. I can add more detail and pics if needed.
Step 1: No Poles No Problem
Find a couple of green sticks (so they are flexible) long enough to extend a foot or so into the sleeves that hold the poles you or your kids forgot.
Step 2: Raise the Roof
With the tent staked to the ground between 2 trees, use a piece of rope (I used a ratchet strap because I had one) and bring it under the 2 "poles" and secure it tightly at the height of the tent. The tighter it is the better since this will be the support of the tent.
Step 3: Sleep
The tent went through a heck of a rain storm that night and stayed dry. Essentially it was no different than if it were set up with the poles.