I got these for my 1st graders (girls) Halloween-night, Halloween costumes.  This would help keep their legs warm and kind of go with their costumes.  I was a little concerned because these are sized for adults, but they worked.

The best part about these is that the elastic is only at the top.  You get quite warm wearing them, so the fact that the bottom is open helps them lay nicely while also giving some air!  Of course, for my twins, these came up to their thighs (covering their legs below their Halloween dresses), so it was perfect.  I think the length is very nice for any height adult as well, and appear to be able to be sized (by cutting off the length).