Line an 8-by-8-inch baking dish with parchment paper and lightly coat with cooking spray. Line a baking sheet with parchment paper.
Combine the butter and marshmallows in a large microwave-safe bowl and melt in a microwave in 15 second intervals, stirring between every interval, until the marshmallows are completely melted. Alternatively, melt the butter in a large saucepan over medium heat, add the marshmallows and cook, stirring constantly, until the marshmallows are completely melted; remove from the heat. Add the cereal and salt to the melted marshmallow mixture and gently stir until the cereal is completely coated.
Evenly press the mixture into the prepared baking dish. Set aside to cool for about 30 minutes or refrigerate for 10 to 15 minutes.
Remove the treats from the pan and slice into 16 squares. Place them on the prepared baking sheet.
Place the dark and white chocolate chips in each of 2 microwave-safe bowls. Melt in a microwave in 15 second intervals, stirring between every interval, until completely melted. Transfer the melted chips to 2 separate piping bags or resealable bags. Snip the tips and drizzle over the cereal treats, then sprinkle with the marshmallow bits. Before serving, allow to set at room temperature for about 30 minutes or refrigerate for 10 to 15 minutes.
Never leave a child unattended in the kitchen. Limit the child to tasks that are safe and age-appropriate.