Make the treats: Line a 9-inch square baking dish with foil, leaving a 2-inch overhang on two sides; lightly coat with cooking spray. Melt the butter in a large saucepan over medium heat. Add the marshmallows and salt and cook, stirring, until melted and smooth, about 5 minutes. Remove from the heat, stir in both cereals until coated, then press into the prepared pan. Let sit at room temperature until firm, about 30 minutes.
Meanwhile, make the icing: Whisk 2 cups confectioners' sugar and the meringue powder in a large bowl until combined. Add 4 tablespoons water and beat with a mixer on medium-high speed until smooth and the consistency of a thick glaze. Add 1 to 2 more tablespoons water if the icing is too thick. Cover with a damp paper towel and plastic wrap and set aside until ready to use.
Lift the treats out of the pan using the foil overhang, then cut into rectangles. Insert a lollipop stick into each. Brush an even layer of icing on the cereal treats with a pastry brush. Add the remaining 1/2 cup confectioners' sugar and the ground cinnamon to the remaining icing; beat with the mixer until thick and pipable. Transfer to a small resealable plastic bag and snip a corner. Pipe thin lines on the cereal treats to look like bandages and add 2 chocolate candies for eyes. Let set, about 15 minutes.