Niagara Falls is a ton of fun for the kids! I don't think I'd spend a week there only bc we've been twice, but I would need at least 3 days to do the minimum my kids would enjoy.
All of the touristy stuff is on the Canadian side.
You can do the Maid of the Mist boat tour.
Cave of the Winds tour.
Marineland is well worth a stop for the day.
As far as the Smokies go, I'm not sure what your interests are, but we were in the Smokies with kids this past April. You couldn't pay me to stay in Pigeon Forge. It's a 6 lane cement heaven, you can't walk anywhere and it's only appeal is the multitude of go-cart facilities.
