Preheat oven to 200°F. Set aside two parchment-lined baking sheets.
Help your child draw nine 3-1/2-inch hearts on each sheet, leaving at least 1-inch between them. Turn over the paper.
In a small bowl, whisk together superfine or baker's sugar and cornstarch; set aside.
In the bowl of a standing mixer fitted with a whisk, beat egg whites on medium-low speed until frothy, about 1 minute.
Increase speed to medium high, toss in a pinch of salt, and slowly add the sugar mixture, whisking until stiff, but not dry, peaks form, about 2-1/2 minutes. Add white vinegar and red food coloring, and increase the speed to high. Whisk until stiff and glossy, about 4-1/2 minutes.
To create each heart, immediately take a heaping spoonful of meringue and place it within one of the two heart arches, swirling it down toward the point. Repeat with the other side to form a solid heart.
Bake in the middle rack of the oven for 1-1/2 hours. Turn off oven, and leave the meringues inside with the door shut until completely cool, 4 to 6 hours or preferably overnight. Store in an airtight container for up to two weeks.