Place white chocolate and Andes baking chips into a microwave-safe bowl. Heat for 1 minute and then stir. Heat again for 30 seconds and then stir. Repeat until chocolate is melted and smooth.
Place Rice Chex and Lucky Charms cereal into a mixing bowl. Pour melted chocolate over cereal and mix until cereal is completely coated.
Transfer coated cereal into a gallon-sized food storage bag. Pour powdered sugar into the bag and seal the top. Shake until the cereal is well coated in powdered sugar.
Transfer the cereal to a serving bowl. Add the Lucky Charms marshmallows and green M&Ms to the cereal and stir to combine.
Serve immediately or store in an air-tight container for later.
Notes
I usually have my kids separate the Lucky Charms cereal from the marshmallows. Little kids LOVE doing this because they usually sneak a couple of marshmallows during the process. :)