PVC pipe and fittings are pretty cheap materials to make your kid . . . or you . . . a kit for building forts and tents. For under $50 you can whip this out with little effort. My inspiration for this was a "tent" I had as a kid. A plastic Transformers set pictured here.
You can get as fancy as you want a make bent pieces with the PVC, but I kept it simple here. If you want bent pieces you'll likely want to break out a heat gun and maybe make a form so that your pieces are uniform.
Step 1: Supplies
I started with 5 pieces of 10 feet long electric conduit PVC. The electric conduit is typically cheaper than plain, white plumbing PVC. We're only talking $0.25 to $0.80 cents each, so if the pieces being all white is important to you, go for the white.
One problem with the conduit is that one end is usually expanded so that you connect them without buying coupling pieces. This means that if you go for 2 feet sections like I did you'll be left with a piece that you either have to cut shorter or find an alternate use for. I may cut my extras into 12" pieces and 6" pieces and keep the expanded end for actually electric conduit uses.
Buy the contractor bulk packages of PVC fittings. They're usually less than $4 for a pack of 10. I haven't found the elbows with side outlet or the cross pieces in bulk. I'm sure you could get them like that online.
Step 2: Tools & Doing It
I love my pipe cutter. Cuts through pex and PVC like it wasn't there. A hacksaw or power tool will work just as well. Cut your pieces into whatever lengths you wish. 2 foot pieces seemed good to me. You can add in some 12" pieces and 4 footers if you like.
Measure your pipe and mark it with a pencil or marker.
Step 3: Building
Push the pipe into the fittings and go. Make sure you tell the kids not to climb on it, of course . . . they'll break the pieces and their heads.
Some helpful additions to your kit could be some clothes pins to hold the sheets or blankets in place. And some LED christmas lights could be a very cool addition to the building process.
Obviously I'm not a master fort maker as you can see from the pictures. You need a wise child for that.
I had an old tripod carrying bag that will make a great case for the set. A large plastic tote would be great too. When done the kids could put it in one and slide it under the bed.