Heat oven to 350. Let the kids coat a 9x13-inch baking pan with 1 tablespoon of the butter; set aside.
In a small pan, melt remaining butter over medium-low heat. Whisk in flour until well blended and cooked, about 1 minute. Add maple syrup, salt and brown sugar. Simmer, whisking frequently, until the sugar dissolves and sauce thickens, about 4 minutes. Remove sauce from heat.
Help kids arrange a row of sweet potato and apple slices, alternating and overlapping them, in the prepared pan. They can continue to make rows until the pan is full. Pour the sauce evenly over potatoes and apples. Cover pan tightly with foil and bake 40 minutes.
Raise oven temperature to 450. Remove foil from pan and use a spoon to baste the juices over the top. Bake an additional 30 minutes, or until potatoes are tender and nicely browned, basting juices over the top every 10 minutes. Serve immediately or keep warm in a 200 oven up to 30 minutes.