Take a 9x13 pan and butter it with your non-stick spray.
Take a large non-stick saucepan and melt your butter on low heat.
Add your entire bag of marshmallows to the pot. Stir constantly on low heat until your marshmallows are mostly melted.
Combine your Lucky Charms cereal with the marshmallows in the saucepan and stir until all of your cereal is coated in marshmallow.
Transfer your mixture into your 9x13 pan.
Spray a spatula with non-stick butter and use it to help spread the mixture throughout the pan evenly. Be sure that there are no gaps and that your mixture is level by lightly pressing down with your spatula. Do not over press because your cereal bars will get too hard.
Sprinkle the top with your rainbow sugar sprinkles and press down lightly into the mixture.
Let your treats sit and firm up for 2-3 hours.
Cut them into squares with a knife and store in an air tight container to keep them fresh.