Grand Canyon or Yellowstone for 2015?

I've done both with my kids, and if I had to choose I'd go with number #2 with 5 nights in Yellowstone and 2 in the Tetons. If you were fairly sure you'll due both trips eventually, then no big deal, but otherwise, definitely Yellowstone. With the young one you won't be doing any all day hikes or trail rides, so Yellowstone will have a lot more to do than the Tetons, in my opinion.