I've seen the bed tents in stores and they are usually pvc like quiltmouse mentioned. Then you could attach them with little fabric loops.

Lowes or most hardware stores have clear plastic tubing for your circle, for the turret of the tower. They have a little piece that enables you to join the ends.I would use two panels and have the back slit the way its shown in the front in the photo.And as several have mentioned hang from the ceiling.Our Walmart here has that parachute type of fabric pretty reasonable.Oh, the fun those girls will have. Brings back so many memories,my sister & I had bunk beds built into the hallway, similar to the ones on the passenger trains and we hung blankets and would pretend we were on one. Good luck.

The tower looks to be made of the parachute fabric mentioned earlier. I've purchased it at Joann's for use as an outdoor banner. It is not difficult to sew (only problem I had was with appliquing the Old English style letters for the name). The idea of hula hoops to create the circle shape also sounds like a very simple and cost effective solution. To do the cone shaped top you can just cut a circle then cut a pie shaped piece out and overlap those edges and stitch. I also like the idea of suspending the tower from the ceiling. You can get one of those hooks like for hanging plants or lamps and attach a hanging loop at the top of the cone to the hook.
I think you can do this fairly easily actually!
Do let us see it when you're done.

I want one!! Sniff... I always wanted a pink canopy bed and never got one. My childhood was SO deprived.... lol!

Thank you all sooooo much - what brilliant ideas! suspended hula hoops it is... and little sister is giving me her list of what she wants inside... they are both getting quite excited and we've had to have a little sit down to explain that this is all going to take a really looooong time to do.

Best I get the birthday/Christmas present list made double quick! Happily everyone wants something homemade (mostly quilted items but some soaps, candles, etc) so I can indulge myself without feeling guilty.