I would think Mayfield would be better. More interactive, getting in and walking the falls. Bring water shoes, but I would think they would love it. A bit closer, as well, but not much difference.

YS is beautiful, but not as much to do. A few places to swim, but not to the level of Mayfield. Great place to see, but I don't think I'd drive that far unless I was going to Pelican Bar or something, which probably wont work for the kids. Not saying they wouldn't like it as well, though. Not sure you can go wrong.