All you need for some cosmic fun are a couple of really cheap, 30 gallon, black garbage bags, with a thickness of .5 MIL (this is not a job for Hefty), some masking or clear tape, scissors, and a cloudless sky. It can be cold, but it must be sunny.
Cut the bottom of the first bag open at the seam.
Overlap it with the opening of the second bag, and tape the two bags together, one inside the other, making sure there aren't any gaps for air to leak out.
Fill the bag, either by swinging it around outside, or with a blow dryer.
Tie the bag closed. You might want to tie on a tether string, just until you're sure your bag is not going to float away. In fact, if you keep adding bags to the end, you'll end up with one of those hot dog shaped solar balloons.
But at this size, the bags and tape should be heavy enough to keep the balloon from floating completely, for the amount of hot air inside. (The fingerprint in the following pictures, is the result of my giving the camera to my five year old to hold, and not checking the lens, when I took it back.)
It will however, hover nicely, mid-air, for a few seconds when bopped around the yard.
Sort of like playing beach ball on the moon (or somewhere with a gravity slightly less than ours).
You'll want to supervise younger children closely as they play - all the normal suffocation warnings for plastic bags apply. And, the plastic is pretty thin, so if they snatch at it too hard, they'll pop it.
When we were ready to pop our balloon, the children discovered if they poked it, without puncturing it, the hot air inside, would puff out their finger marks, which was great fun, too.
If you actually want to fly a solar balloon, we've found Solar-Balloons.com, to be the best website for DIY step-by-steps. We've had some real luck with their tetroon design. In fact, you can read about our flying, and losing one (kind of a cautionary tale) last year, by clicking here.
It's great to be a homeschooler.