Magical quality about them or magical quality to them?

Both are, but depending on the context one may be preferable to the other.

Izzy is right of course, as always; though any differences are subtle

So I asked my Better Half, who's much smarter than I, and she prefers "about"

Izzy's correct. They're almost interchangeable, but context matters a great deal. I would say that the native speaker of English would unconsciously use one or the other based on the context.

There's a magical quality about the desert at sunset, with the rich, vibrant colors that change as the sky deepens in color to night.

Or, alternatively:

I'm tellin' ya kid, you wanna trade that cow for these here beans. They got a magical quality to them that you just ain't gonna believe.