Bring the apple juice to a simmer in a medium saucepan over medium heat. Toss the Gruyere and cornstarch in a medium bowl. Add to the simmering juice by the scant handful, whisking constantly until smooth and letting the sauce come to a boil between additions, about 30 seconds. Remove the saucepan from the heat, stir in the Parmesan and mustard and season with 1/2 teaspoon salt and a few grinds of pepper. Pour into a serving bowl, and serve with the dippers of your choice.
Never leave a child unattended in the kitchen. Limit the child to tasks that are safe and age-appropriate.
Copyright 2015 Television Food Network, G.P. All rights reserved.